There have been several hints in the past, including these two of mine, with regards to using push email on the iPhone, or using MobileMe with custom reply-to entries, etc. A new application called PushMail ($5; App Store link) seems to me to be the ideal way of using push email (or any mail) on the iPhone. (I am not affiliated with this app at all; I've just been looking for an ideal iPhone push solution for a while, and this seems like it.)
Essentially, PushMail gives you your own new email address, something like a@dopushmail.com. Everything that gets sent to that address has a push notification displayed on your phone. The notification displays the sender, the subject, and as much of the email as can fit in a notification. So, you can configure your regular email addresses to all forward a copy of your email to that new address, and you will have your email pushed to you. This is an option in the settings for most webmail email services such as Gmail, or you can configure procmail to forward a copy, if you've got control of your email server.
Then, in Mail.app on the iPhone, you can set your email to refresh manually, or at some infrequent interval. I've got mine set to refresh every hour, just so that it downloads messages automatically every now and then. I've also turned off the 'new mail sound' for new mail, so that I only get notified of new mail from PushMail rather than Mail.app.
The advantages of this over forwarding mail to MobileMe is that you can reply right from your regular IMAP or POP email addresses instead of a MobileMe address, solving the problem of having to hack in a custom reply-to address if you don't use your MobileMe address as your primary address. Also, the notification means that you do not need to open Mail to see a summary of the message. This, to me, is also a huge improvement over having to turn on the phone, open mail, and open the message to see the message. It makes it akin to GrowlMail on the Mac, where you can decide whether or not you want to read the message now based on the summary. Another advantage of this app is that you don't need to give your login credentials to a third party for all your email accounts. Just forward a copy of the messages to the new address.
Lastly, the application is well designed in that it does essentially nothing. It does not get in your way. With most apps that push, if you get a notification, and turn on the phone while the notification is on screen, it will directly open that app. This app, however, only goes to the home screen, knowing that you won't want to open PushMail but will instead want to open Mail.
[robg adds: We don't normally feature pointers to iPhone apps as hints, but push email and the iPhone have been a popular subject around here, and this looks like a reasonable solution for those looking for something better.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090806101143257