In November I gave the beta exam for silverlight 4, it took me more then four hours to complete the exam questions, review and comment. For beta exams you do not get the score right away, exactly after three months I got an email from Microsoft stating that I passed the beta exam for silverlight and now I am certified Silverlight 4,Developer.
Silverlight exam is now released and is available through prometric testing centers and the exam number is 70-506. I like beta exams because they are challenging, one reason is you don't get ready made material to study all you have is MSDN documents, blogs,some samples and forums. There is a huge community support for silverlight and luckily there where some good books in the market. I find many of my fellow programmers worried about not able to find definite material to prepare for the exam. I am going to put together a list of useful links that will help to prepare for the exam.
Exam details on Microsoft learning.
Silverlight Show Getting ready for silverlight exam 70-506
Microsoft training catalog
Silverlight Documents on MSDN
Silverlight 4 Hands On Labs
List of all popular books
Chanel 9 - What's New in Silverlight 4
Videos and screen cast
For developers having right experience the exams are not really tough but I would say the questions are tricky so make sure you have good hands-on experience with silverlight before you appear for the exam. Although the exam focuses on sivlerlight 4, I would say Visual Studio 2010 and Expression Blend are inevitable tools for a silverlight developer. I have tried to put together some of the good links, if you find some more good links please put them in the comments