Time Machine - older backups gone - why?

I have a new (Dec '08) MacBook Pro, and I bought a 1TB G Drive Q external HD to go with it, for backups. Time Machine started keeping backups on December 13. Until today, there were no problems - backups were added and visible in TM as expected.
I normally use my MacBook in a different room, away from the desk the HD is in. Every week or so I take the MacBook to the desk and hook up the HD so it will do a backup. That's what I did today. However, after the backup, all older backups seem to be gone. TM only shows the 1/26/09 backup, all the tabs going back through time before that are blank. TM Options lists the oldest backup as 12/123/08. The HD is far from full - still 781GB free.
What gives? Am I missing something about how TM works? Why would the backups go away?

Can you see the backups via Finder? I had the same problem and I could see all of them - a year's worth - in Finder yet I could only see 1 days worth in TM - major panic! Once I'd opened a backed up file via Finder all of the backups re-appeared in TM. Don't ask me why - so might be worth trying. TM behaves oddly at times.

Thanks, Terry C! That did the trick. I just went into finder and clicked on a folder or two - didn't open any files or anything, just viewed the folders in finder - and now Time Machine shows all of the backups.

Thanks guys... had been searching for an answer to related question for ages. I started time machine over wireless network originally, then moved to wired to increase speed, before moving back to wireless for ongoing use... could always see the backup files in finder but no cool GUI for me :-( tried ur advice to no avail but then reselected the same external drive as my time machine over the network and now all my backups are visible. may help others who end up on this page!