I have exactly this same issue. The problem for me is compounded by the fact I have so many not getting virus defs, and due to the fact these clients are scattered all across the country. The number of clients I work with is also large, so manually deleting virus definitions is out of the question.
Right now my Home page shows the following:
2010-09-13 rev. 048 2800
2010-09-13 rev. 024 1722
2010-09-13 rev. 004 295
2010-09-12 rev. 005 151
all others 4623
Now before you say "Well, the rest are maybe a day or two out", let me assure you this is not the case. Looking through the list, I see 175 clients on 09/02/2010 rev. 50. If I go farther back, I hvae 37 clients on 07/26/2010 rev. 41. And then I have still 28 clients on 04/15/2009 rev. 33.
As the person above has done, I do know these clients are checking in. They are in groups where other clients are updating. Many are in the same store where other clients are properly updating, but maybe one client in the store doesn't update. The policies within a store of course are exactly the same, same GUP, etc.
This has been an ongoing issue for us, I just haven't had the time to research it until today.