Some useful links with personal impressions, valuable resources or important information and their date posted. Don't forget, always check the preparation guide (last update 2007-03-20), both as a road map for your preparation and just prior to taking the exam (maybe things have changed since you started).
The links prior to 2007 don't include info on the simulation questions. The first time I heard about the simulation questions was in December 2006, it seems they were running beta into March. Starting March 2007 I heard and read about failing exams because of the simulation questions, which seem to make up a very big part of the overall score for the exam.
Must read is the discussion on simulations (2007-04-12) started by Zieglers after he passed his 70-431 (2007-01-23). Some very valuable links can be found in the discussion, including this simulation example.
Also check out the database certification newsgroup microsoft.public.cert.mcdba. This newsgroup is aimed at all SQL Server exams and certifications, not just 70-431.
Other links:
If you use the MCTS Self-Paced Training Kit (Exam 70-431): Microsoft® SQL Server™ 2005—Implementation and Maintenance, check out the comments and corrections.
Please post a comment if you have other good and free resources as well.
*UPDATED 2007-05-05*