After experiencing this problem with the 4GB Kingston microSDHC I searched around and came across this XDA-Developers page. To summarize, if you have a problem with the disappearing memory card try increasing the values in [HKLM\System\StorageManager]"PNPUnloadDelay" and [HKLM\System\StorageManager]"PNPWaitIODelay" registry keys. The default on SCH-i760 is 4096, I doubled it to 8192. It's been a few days since I applied this fix and have not had a missing card yet.
For more registry hacks and links to some registry editors, check this site.