Short answer: Mid to late 2006.
[from 1/3/2006]
If there are any announcements of an ATSC/Cablecard model that will work with over-the-air (OTA) or cable broadcasts, they will most likely occur this week at the Consumer Electronics Show (CES) in Las Vegas, on or after January 5th.
Their past MO has been to announce in January at CES and then release later in the year (April, May, June).
Tivo's official press release site for CES is: http://www.tivo.com/0.9.0.asp but there is no news there yet.
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[from 1/6/2006]
Some News on Tivo is now available from Yahoo News this morning:
TiVo announces Series 3 HD TiVO, due this year
http://engadget.com/2006/01/05/tivo-announces-series-3-hd-tivo-due-this-year/
TiVo shows Series3 HDTV Cable Card unit at CES
http://www.tivolovers.com/252572.html
TiVo unveils Series 3 HD DVR
http://arstechnica.com/news.ars/post/20060106-5915.html
Reported dates are "mid-to-late 2006".
Other things demo'ed are set-top information services in alliance with Yahoo!:
http://news.yahoo.com/news?tmpl=story&u=/060106/ids_photos_ts/r2631397219.jpg
http://www.engadget.com/2005/11/29/tivo-and-yahoo-bring-some-new-functionality-to-your-dvr/
As for why no big press releases:
http://engadget.com/2006/01/05/tivo-cancels-press-preso-for-informal-product-demos/