You guys are actually having the issues stated over on this thread:
http://forum.androidcentral.com/lg-optimus-s/43684-freezing-lock-ups-sd-card-issues.html
In summary: It seems they are finding that many SD cards that the phones are coming with are formatted improperly, and they are reformatting to FAT32 with success. Make sure you are always Safely Removing the phone from your computer before you unplug it.
These fixes never solved my problem (I was having the same issue) so I bought a new 4gb Class 4 Kington Micro SD card and have never had an issue. Some info that I gained from reading various different forums and bug reports on the issue (with other Android devices) have come to the conclusion that Class 2 SD cards have a tendency to cause problems with Android, and that Sandisk and Samsung cards also tend to have issues. This is all from a personal standpoint, and the formatting fix may solve your problem, this is just an alternate to try if you have not had luck formatting.