Apparently TiVo has been giving up some details about the Series 3 today—no official announcements, but the writer at Megazone has somehow gotten the scoop. It's an HDTV unit with two CableCARD slots in back, and a window that shows what you are watching/recording while it's at work.
Yes, the unit is dual-tuner - actually, like the HD DirecTiVo it can use any two of the tuners it has, and it has six. 2 cable tuners, 2 ATSC tuners, and 2 NTSC tuners. Yes, it supports digital and analog cable, digital ATSC OTA, and analog NTSC OTA.
The remote has been upgraded, too. I'll get over to TiVo first thing tomorrow and update with more info.