Depends on the kind of DVR... If you're thinking TiVo-esque with it's own cable decoder and taking cable cards, it's more likely they'd need a hardware update than having some extra device you'd buy and plugin to an existing AppleTV. But it seems like they could choose to make the AppleTV eyeTV compatible or could go the extra piece of hardware direction. I just think with Apple's integrated design approach, it's more likely there would be a substantial hardware refresh.
This, of course, assumes Apple will ever build a DVR. With iTunes rentals, TV show sales, YouTube, and the continual increase in programming available online and on demand, DVRs only have so many years left until the concept of recording TV is moot.