Matt, I've experienced two Silverlight related quirks recently:
On a new (well about a month old now) clean installation of WinXP SP3 IE8 has ceased to view any Silverlight site. The plugin is listed and enabled but every SL site asks for installation. I've uninstalled|reinstalled both SL3 and IE8, in turn, to no effect.
Curiously, it has worked flawlessly in FF3, Safari 4, Chrome 2, and Opera 10 (well, there are a few curiosities in Opera, but not enough to break the experience) both before and after the uninstall|reinstall.