iBloggedThis has the scoop on Comcast to begin testing HD TIVO service this summer. The deal was announced over a year ago and it sounds like some form of the TiVo OS will download automatically and power the Comcast 6412 unit.
The AVS forum set up a space specifically for this so first reports from the wild will likely show up there. No one is sure what the extent of TiVo functionality will show up on Comcast hardware, but everyone seems to be aware that the upcoming Series 3 TiVo will compete with it, which puts TiVo in the odd position of competing against themselves.
I'm going to get a Series 3 box as soon as humanly possible, both for the Over-The-Air (OTA) HD recording and the full functionality, but it'll be interesting to see if the 6412 experience improves with TiVo software added. I imagine without the tightly integrated remote control, it won't be the same as a real TiVo, but if it can solve the screen lag and software lockup problems Comcast boxes currently suffer from, it'll still be a huge improvement.