This just happened to me, and I'm wondering if anyone else has experienced it.
I've had 2.1 for a week (I think) and no problems. I did a factory reset just to be sure, and then set everything back up the way I had it (apps, ringtones, etc).
So just five minutes ago I'm talking with my brother via BT, and when I go to hang up, the phone is at the "You Must Activate This Phone" screen, just like it was brand new. So I went ahead and did what it said, and it finished with "your phone is now activated" (or whatever it said - I really wasn't paying attention).
How the heck could I use it if it wasn't already activated?