Microsoft learning has announced that after August 31st, 2007 only Thomson Prometric will be allowed to conduct Microsoft Certification exams. Pearson Vue can take registration for exams till August 31st,2007 and can conduct the tests till December 31st,2007. In case you have any Pearson VUE vouchers its the time to book your registration for the exam.
The official details are here
I have recently taken exams for WSS 3.0 and MOSS with Pearson Vue and have taken exams with Prometric as well. I personally think that Pearson VUE was much better in conducting the exams. There customer service was much better and much less problems then Prometric. Being a MCT as well thats what I have heard from my students as well.