The official Microsoft guidance, at:

http://www.microsoft.com/learning/mcp/sql/2008/default.mspx

says that if you are currently working towards your MCTS for 2005 (i.e. exam 70-431), then you should complete that - and then upgrade your qualifications to 2008 when required.

I would agree with this. If you are currently working with 2005, study and take exam 70-431. If you are not currently using 2005, haven't started studying yet, and just want to gain a qualification, then you might want to jump to the new 2008 exam.

Andy