I seem to remember hearing about something along this lines on early builds of iPhone OS 3, but I'm guessing the chances of that being the cause here are pretty low.
Ummm, just for completeness' sake, can you open
Settings.app -> General -> About
on your phone and let me know what version of the OS is installed? (I'm assuming iPhone OS 3.1.2 is installed, but it never hurts to check.)
If that doesn't turn out to be the case, we'd definitely like to look into it; if you
send us email or give us a call at 800-315-OMNI (6664), we can gather some more information and figure out what's going on here.