I just read on the TiVo forum that Steve Thomas is being replaced by Kevin O'Connor, the participant of an ask TOH segment.
While I was no fan of Steve Thomas in the beginning I grew to like him. Steve was also a much more accomplished person than Bob Villa who was removed from TOH after doing commercials for Sears.

Stories here:
http://www.thisoldhouse.com/toh/html/news/newhost.html
and here:
http://www.nydailynews.com/entertainment/story/89516p-81462c.html