So:

1. You can sync google calendars to your iPhone.
2. It utilizes Microsoft Exchange so you can get the changes pushed to your phone, and it's fairly quick.
3. You will be automatically reminded of the event a day before with a popup.

Therefore:
If you allow people to post to your calendar, and they post an "event" with a message for you to receive, you will get it even if there is NO AT&T service (just WiFi)!!!

Example: You're in a conference or office with no service for AT&T at 02/24/09 @ 1200, but you have WiFi access. I want to send you a message. I add an event to your calendar with an event start time of 02/25/09 @ 1202. The Microsoft Exchange Sync will push the change to the phone and at 02/24/09 @ 1202 you will receive my message. An alternate free SMS service in a way. Interesting.

There have to be more hacks along these lines.