Another day, another rumor about a change to Doctor Who's format for the first year of the Steven Moffat and Matt Smith era. The latest news suggests you may have to wait until late 2010 for the second half of series five.
Apparently, there are rumblings that spring 2010 will only see the transmission of the first six episodes of the eleventh Doctor's debut series. The show would then take a break until around October, at which time the BBC would broadcast the remain seven episodes. A Christmas special would then close out the year.
If I had to guess, this could have something to do with the World Cup, which will take place from June 11 to July 11 in 2010. The BBC might well remember the last episode to come into conflict with a big World Cup match. The series two episode "The Satan Pit", which faced off against England's first match in the 2006 tournament, was the lowest rated episode of the new series. The move to split the series in two might also be meant to avoid the more general ratings drop Doctor Who experiences in the summer, as the weather gets nicer and more families spend Saturday evenings outside.
On a less pragmatic and more creative level, this could allow for the series to put in a truly massive, months-spanning cliffhanger at the midseason break, something the series has never really done before. Generally speaking, the need to preserve the Christmas specials as events unto themselves has meant they had to be separate from the stories before and after them, precluding the possibility of a big cliffhanger at the end of either the previous episode thirteen or the special itself. (Admittedly, "Doomsday" and "Last of the Time Lords" threw in random teasers for their subsequent Christmas specials, and "Journey's End" almost did the same. But those were all incidental to the story that had preceded it and neither of the two that actually aired placed the Doctor in danger.)
There's something wonderfully cruel and perverse (and thus very Steven Moffat) about the thought of ending episode six with Matt Smith and Karen Gillan in mortal peril from some terrible enemy, only to suddenly cut to the words, "See you in six months!" Frankly, I'd be disappointed if Moffat didn't do it.