For a while now, I have been looking for a way to keep my iPhone calendar in sync with my work Microsoft Outlook calendar. Now I've found a way!
Part 1: MobileMe, iCal, and iPhone:
At home, I sync my iPhone calendar with iCal on my Mac. Now with the new MobileMe, my iPhone stays in near constant sync with my MobileMe calendar. My Mac at home also stays in sync (syncs every 15 minutes) with my MobileMe calendar.
Part 2: Spanning Sync - iCal and Google Calendar
There is a program called Spanning Sync that syncs iCal on my Mac with my Google calendar. This program also runs every 15 minutes. (The downside is that this program costs a whopping $65.)
Part 3: Google Calendar Sync - Google Calendar and Microsoft Outlook
Google has released Google Calendar Sync, a program that syncs your Google Calendar with Microsoft Outlook. This program also syncs every 15 minutes.
Of course, these syncs aren't immediate, but now you don't have to enter your calendar items two or three times. I know this all seems very complicated, but it works! If you've been looking for an Outlook/iPhone sync solution, give it a try and post your experiences (or alternative solutions) here. Good Luck!
[robg adds: I haven't tested this one.]
Mac OS X Hints