Cory Doctorow, that infamous SF author, blogging guru, and overall technology person, has written another article for Locus Online - called "The March of the Polygons: How High-Definition Is Bad News for SF Flicks." Instead of restating his points, I will merely point you to the article. So...here you go.
It's looking that way. See Steven Bagley and Rob Ford's posts. Most of the buzz surrounding Torchwood seems to confirm that that series is still being shot in HD, though.
A shame about Who, if it's true. Still, Series 4 will almost certainly be HD.