USB Drive Will Not Mount, Not Listed in Disk Utilities, but Found In System Profiler
I just started having this exact same problem with my FreeAgent Go drive which has always been working perfectly.  Like the original poster I cannot see the drive in Disk Utility but it does show up in System Information.  After a little more digging I found that the OS is trying to repair the volume BEFORE letting Disk Utility see it via an process named "fsck_hfs".  When I force quit that process in Activity Monitor (it's owned by Root so you'll need to give it permission), the volume immediately appeared in Disk Utility.  I'm running a repair on it now - fingers crossed.  My guess is that since this is formatted HFS+ Journaled something about the journal consistency got flagged and the OS is attempting to repair it from the journal.  I'm shocked that they hide it from Disk Utility while it's happening though - much better would have been to display the drive there without mounting the partition, and display a message that this is being done.  Oh well at least I can see why Windows would mount it but OSX won't now (any OSX machine would start performing the same repair process).