Jeremy Alles publish great article trying to compile existing WPF MVVM frameworks mentioning which of them can be used in Silverlight.
http://www.japf.fr/2009/10/a-quick-tour-of-existing-mvvm-frameworks/
Microsoft UI composition Framework aka Prism is missing there, but it will be included soon. Article contains framework comparison by listing their feature lists.
Pretty big list to compare. Kudos to Jeremy.