Given the improvements were seeing with the latest cameras from both Nikon and Canon, I'm of the mindset that NO, addition and enhancement of video features is not having a negative impact on still photography capabilities. The sample images from the D800 and 1D X are phenomenal from an ISO and noise standpoint, and other aspects such as color, contrast, etc. look excellent as well.
Logically, I'm still in the NO camp. Sensor design keeps improving...higher densities, lower electronic noise levels, better quantum efficiency, higher frame rates, better shutters, more features, better AF systems, etc. etc. Unlike film, digital brings to the table the opportunity to use the same exact hardware for additional purposes. Video features are pretty non-intrusive in the cameras that offer them, and they simply use the same hardware in a different way, maximizing the capabilities of the whole camera. I would actually argue that use for video has helped spur on the ISO wars again...pushing Canon to make usable ISO at 51,200 levels possible because its just as helpful on the video front as the stills front. I think native ISO 51,200 from Canon was first seen on the 300C, and I believe Canon made headway on ISO performance by working on video...it forced them to evaluate the problem and look at it in new ways that resolved issues on both fronts (btw, ISO 20,000 and above on the 300C looks FANTASTIC, and while I don't expect it to look quite as good on the 1D X given the nature of still photography, it should still be an order of magnitude beyond what we've seen before.)
I think video will, long term, be beneficial to still photography DSLRs. It will spur on new levels of competition between manufacturers, all of whom very well know that their still photography customers are by far their largest base of customers for DSLRs, and will probably remain that way for the foreseeable future. They won't do anything to compromise that, and with more heated competition to bring quality still and video features to key cameras in their lineups, I think that could only be a good thing for the consumer...not a bad thing. And if it becomes a big enough issue for enough of their DSLR customers, I'm sure manufacturers will start selling camera models that only offer still features and eliminate the video features (although I don't think that would have any impact on the quality of stills you could get from such a camera...it would ultimately just boil down to appeasement.)