I sent an email to the ted.com folk a few weeks ago. They said that TED talks aren't on TiVo any more, but they are working to get them back.
It seems that there were some video format issues when the TED talks were viewed on TiVo (I can vouch for that, as some of the shows were not viewable), and ted.com is trying to encode the video in a manner that is agreeable with the TiVo units.
No date was offered for the return of TED talks to TiVo's web video service.
It probably wouldn't hurt to send a nice email to ted.com. Here's
a link you can use.