I find this thread very interesting.
I have been having this problem for some time now. It started as a "preparing sd card" notification. Pulling out and reinserting the sandisk sd card, making sure it was seated properly, stopped that issue. Later however it started erasing/losing its formating.
The interesting part is that I of course called the mob's (t-mobile) troubleshooting dept. They told me they had no record of known issues with the sd cards. This was just two days ago!
I let them know what the problem was and what steps I had taken to try to resolve the issue. They agreed to send me a replacement phone. It's on its way.
I of course had downloaded apps from the market. When I was on the phone with the mob I was asked if I had done a factory reset. I had not. Because I did not want to lose all of the apps. Of course since I can re-download them anytime even the paid apps, I decided to give it a shot. That was two solid days ago and so far the problem has not come back.
I have put some but not all of the apps back on my phone. I intend, one at a time, each day, to add back an app to see if the problem returns. If it does I will see if it is repeatable and post the problem app if I find one.
Back up your contacts, factory reset the phone and let us know if it works for you. Test each new app you dl for at least a day before adding a new one.
****Update****
My replacement phone got here before I had another problem. My replacement has not shown a problem either.
****Update****