From the TiVo Help Forms, courtesy of "TiVo Pony," whom I gather is somehow affiliated with TiVo (and not just some crazy message board discussion artist):

Perception is a tricky thing. To you (and admittedly to us at first) this appears to have cropped up out of nowhere. No software updates, no changes...just something weird that started happening. Now that we understand the issue it turns out to be quite predictable.

There's a rather rare circumstance that can happen when the box is allocating disk space for the end of a recording. Doesn't happen to everyone, and only happens if the box has been up and running without a reboot for a very long time (roughly a couple of months). The result is the shortened recording reported here. We do have a fleet of long-soak boxes (boxes that just run and run and run without reboot), but as the occurance is somewhat rare we didn't see the issue. Remember that the fifty or so issues reported here and elsewhere on the net (yes, we count them all) are highly concentrated...the vast majority of our customers haven't seen this happen. And our beta folks, good testers that they are, wouldn't have seen this. We regularly send beta testers new software, which reboots their boxes as part of the install process. If you reboot, even just once every couple of months...you'd never see this issue (even if you didn't, the odds of seeing it are still pretty long).

In any case, it's been identified and fixed in the next release. You should have it before another reboot is necessary.

Thanks to everyone here for the detailed reports...it really helped us track this down quickly!

Cheers,
Pony