This issue is fixed on the optimus S as of the VC software release. It was an issue with the code and low class cards. After VC, even the cheapie card the phone comes with stays mounted.
Glad to hear that the VC update will fix this problem. My SD card has been unmounting even with an 8gb class 4 card that's been reformatted (two-three times). Looks like it's time to bite the bullet and update...