The current plan is that there will be three Bank Holiday specials for 2009, and possibly a Christmas special. As recording three (or four) specials takes up less time than fourteen 45 regular episodes, this still allows Tennant time to go do serious actor type stuff*.
A full series will be shown in 2010. RTD has suggested that this approach, occasional 'gap years' to relive the pressure on the crew and actors, may be common in future years. Whilst less Doctor Who in some years seems like a bad thing, it may secure long term production.
*It has been claimed in some quarters that Chris Ecclestone wanted to do more serious actor stuff.
He's now playing Destro in the upcoming GI Joe movie.