A quick Google search brought this up:
http://www.tipb.com/2010/06/21/switching-android-iphone-4/
Moving over contacts, calendars, and email
Heres the good news: if youre using Android chances are youre using Gmail, Google Calendar, and Google Contacts for your personal information, and those all work really well on the iPhone. You can set them up either as an Exchange account (which uses Google Syncs ActiveSync license) to push everything straight from the mothership to your iPhone.
Since iOS 4 can handle multiple ActiveSync accounts, thats the way we recommend you go. Just tap the Settings icon on the Home Screen, tap Mail, Contacts, and Calendars, choose Exchange, and enter your credentials. Google even has a help page if youre not sure just what exactly to put where.
About the only things youll miss and we miss them too is a more Gmail-like mail app. iPhone Mail is a great IMAP client but Gmail does things their own way, with threads and labels and stars. Youll get threads, but the other two just arent there.
If it bothers you enough, you can load up gmail.com right in the Safari web browser. Google makes the best web apps in the business and they work great in Safari (which shares the same WebKit root as Googles own Chrome). Once youve logged in to Gmail, Safari will ask you to cache a few megs of mail on your iPhone HTML5 and SQLite power! and then youre good to go.
On the off chance you dont like ActiveSync you can hit the handy Gmail button instead in Mail Settings and go the IMAP route. If you dont even use Gmail, you can setup MobileMe (Apples expensive push service), Exchange, or pretty much any POP3 or IMAP service you have via the Other button.