The above image is reportedly what the lower end models will look like, and the specs on new models have been bumped up impressively, with two different 4 tuner models and one 6 tuner top end recorder. The hard drive sizes look like they will go from 1Tb to 3Tb in size, both of which are ample recording capacity for HD. Rumored release looks to be in the Fall of this year.
It sounds like updated chipsets will offer a lot more processing power for better software functionality, streaming possibilities, and (finally) built-in WiFi. Personally, this might be the first TiVo box I don't pre-order on day one (as I have with previous versions going all the way back to the series 2 box) since my 4-tuner, 1Tb TiVo XL Premiere is pretty rock solid and working fine for my needs. But if the added computing power makes for more interesting applications and uses for the box, I could be convinced to try the upcoming iterations out.