Apple made it quite easy and intuitive to quit out of apps in iOS 4; however I have not seen this method mentioned anywhere.
Double tap the Home button to bring up the multitasking applications view, then tap and hold on an app (as if you were arranging your home screen). A small red circle will appear on each app; tap one to close that app. And, of course, you can still swipe through the list to see more.
[crarko adds: I can't test this one on my iPhone 3G. Please confirm that this works.]
These are not my creation, but I've found the tips in this webpage very useful for mastering iPad (and iPhone) typing. Full credit goes to that site.
There is a list of ten useful tips for making efficient use of the onscreen keyboard(s) in iOS based devices. Some of them are generally known, such as the URL autocomplete and spellchecking completion, but others are less widely publicized, such as the final tip about toggling the alpha and numeric keyboards.
[crarko adds: I tested some of these on my iPhone, and they worked as described. There are some screen captures and a video on the referenced web site which shows how these tips appear. I'm putting this up because keyboard shortcut hints are generally useful things, but I refer you directly to the original site to preserve their copyright.]
I've been noticing for weeks now that several of my repeating appointments are showing up at the wrong time on my iPhone's Calendar application, even though the right time shows up in iCal on my desktop. The appointments are showing up on the right day, but many hours earlier, e.g. 4:00 AM.
In iCal on my Mac, I opened one of the events that was showing up incorrectly on my phone. Lo and behold, the 'Time Zone' setting was set to floating, rather than to my local zone (North America Central Time). Apparently this was true of all my repeating events that had been around for a long time, presumably before I upgraded to Leopard or Snow Leopard and iCal included time zone support. Just set the time zone to the appropriate region, make sure to change this for all future events, and all should be well.
[crarko adds: I looked in iCal on my machine and all the time zone information appeared to be correct, so this may, as implied, be the residual result of multiple upgrades to iCal.]
I text quite a bit and I tend to have a lot of different groups that I message often (groups of specific coworkers, parents, friends) but not often enough that they stay on top of my 'recent recipients' list. I usually end up adding all the recipients to a brand new group chat and sometimes I forget to add someone.
I've started messaging a unique phrase in each group I have. So for my friends Kyle, Matt, and Johnson I type 'kmj.' Now that you can search message history in iOS 4, I just search for 'kmj' to bring up the previous group SMS.
If you don't want your friends to receive this message, send it in Airplane Mode. The message send will fail, just never bother resending it. It will still be searchable.
[crarko adds: I haven't tested this one. Seems simple, but practical, like Twitter tagging.]
I searched everywhere for a method to get HD video onto my iPhone 4 for use in building videos using iMovie on the phone. I like to have a library of trailers and intros I use in my presentations available on the iPhone if I am building a video while away from my computer. I couldn't find a method anywhere for doing this task -- importing movies via iTunes won't let you import HD video and the content you import is not available in your global iPhone Photo/Video library.
Many websites use special 'Apple touch icon' image files that allow home screen bookmarks to look good alongside app icons. However, Apple’s original specification of a 59 x 59 pixel icon now looks rather low-resolution on an iPad screen or an iPhone 4 Retina Display.
Perhaps Apple themselves were aware of the need to future-proof icon sizes, or perhaps they updated their files while developing higher-resolution iOS device screens, but the Apple site has a larger, 129 x 129 pixel home screen icon that looks much better than the smaller versions. With the help of an old hint from the time of the first iPhone, you can create your own high-res home screen icons too.
While custom Wallpapers are not supported on an iPhone 3G, it is possible to add custom made Wallpaper to a jailbroken iPhone 3G running iOS 4. Here's how.
Following are the requirements (achieving some of the individual requirements is beyond the scope of this hint but can be easily be found on the Internet).
An iPhone 3G running iOS 4 WITH jailbreak
Wallpapers activated during the jailbreak process
Some way of copying files to the iPhone (I used ssh access)
A .png file with a resolution of 480 * 320 pixels (Preview can convert .jpg files to .png)
Once you have all this in place:
Name the .png file 127.png (or if you have multiple files name them 128.png, 129.png etc.). And make a copy of the file named 127.thumbnail.png, and then copy both files to the iPhone into this directory: /Library/Wallpaper/iPhone/. You can experiment with making the thumbnail images smaller if you prefer.
That's it, now when you enter the Wallpaper menu on your iPhone your image will appear.
[crarko adds: I haven't tested this one. I personally do not endorse jailbreaking your iPhone but I understand many people have done it.]
If you have an original iPhone (the Edge only one), it came with a dock. This dock fits the new iPhone 4 and still charges it and seems to hold it pretty nicely. So if you were looking at buying the new dock and still have one of these older ones around, try it out first.
[crarko adds: I haven't tested this one. Given the difference in thickness between the two units this is a lucky find.]
On the iPhone 4, the Hold button found on previous versions' calling screen has been removed to make room for a new button for initiating FaceTime video calls.
Apple has reportedly suggested that using the Mute button is the same, but as noted in the User Guide for devices running iPhone OS 3.1 (on page 50), the Mute function silences the user's own voice in the conversation while continuing to allow them to hear the party on the other end of the line. With the Hold function neither party can hear the other.
Fortunately, if you are looking to make use of the Hold functionality that seemed to have disappeared, a commenter on the TechCrunch piece above notes that simply holding down the Mute button for 2-3 seconds will activate Hold.