Let the hive mind of Engadget get that for you.
"I just switched to Sprint from Verizon about three months ago for the Pre. Then I went for the Hero about a week ago. Now, I miss my hardware keyboard and am thinking about switching to the Moment. I am still able to switch back to Verizon if I want and get the Droid when it arrives. Should I just trade up to the Moment when it comes out, see if I like it, and if not switch to the Droid? Or something else entirely? Help!"
It's your visual voicemail. Nothing AT&T or Apple can do about it - but they should have warned you. Visual Voicemail on the iPhone is different than regular voicemail. It completely resets your old voicemail. It deletes all your old messages, resets all settings, and you start from square one.
AT&T is telling people 2 weeks to prepare you. It could be 2 days, but it could take up to 2 weeks.