April 3rd
There will not be a new series of Doctor Who until 2010 when the new Doctor will have taken over from David Tennant. Planet of the Dead is the first of four specials that will air in 2009 instead of a full series.
2011.
Doctor who started on the first Sunday of May 2010 on Prime. The new doctor is played by Matt Smith. The show starts at 7:30 and goes to 8:30.
If you mean 'why does the next full series start in 2010', then that is because in 2009 they are having four specials instead of a full series because of the BBC budget and the change over of control on the show, they have decided to have specials, and return with a new series with Steven Moffet at the helm in 2010.
September 21, 2010
The filming is well underway in Cardiff with the new Doctor, Matt Smith. The 11th Doctors return is planned in the Spring of 2010 The new Doctor Who series will premier April 3, 2010 in the UK and April 17, 2010 in the US
Series 5 will probably air around Spring 2010.
2010
The first half of the series will start on April 23rd, the other half will start in September.
There are three more specials in 2009/ early 2010, with a full 5th series in 2010.
Doctor Who is back at Easter with the first of the 2009 specials. Three more specials will follow throughout the year, and the next full series will start spring 2010.
Karen Gillan will play Amy Pond in the 5th series of Doctor Who in 2010.