This topic has come up a few times. Silverlight tries to delay rendering until something is ready to be displayed, and indeed calling Measure() and Arrange() is a possible workaround.
For another example, see my post in this thread:
http://social.msdn.microsoft.com/Forums/en-US/wpdevelop/thread/193278c4-ad1c-410d-94e4-741d570cdd38
Richard Woo