For the sole developer out there using Microsoft technologies and not following Scott Guthrie's blog: Silverlight 2 has a unit testing framework.
You can download a project template here:
http://www.jeff.wilcox.name/2008/03/31/silverlight2-unit-testing/
and read a further tutorial on how to use these tests here:
http://weblogs.asp.net/scottgu/archive/2008/04/02/unit-testing-with-silverlight.aspx