I have discovered that when I turn my phone off and then back on it basically resets itself, apparently deleting my apps (though it shows some funny total/available storage numbers in the "about" section of the settings menu) and settings, including any accounts linked to the phone. If I try to reset my phone through the settings menu it doesn't really change anything and my pc also will not recognize my phone. I have to turn the phone off and then hold the down-volume rocker, the camera button, and then the power button until after it displays the Samsung logo. Then once I have confirmed reformatting the phone a couple of times and it restarts it is as good as new, though I have to set everything up again of course.
Had the exact same problem, when I first got the phone, I installed a transcend, class 6, 16gb microsd. Set up everything the way I liked and then after the battery died and it reset itself deleting all apps. set up again and the same thing happened. after that I shut it down, took the card out, and reset with the camera button, lower volume and power button all pressed without the card. Set everything up again and have not had any problems. Some people have been having problems with microsd. 8 gb is fine for now and when microsoft figures it out will install one again. Not that hard to set everything up all over. Make sure you sync with your zune software though