Nasty upgrade bug with .NET 3.5...
or silverlight, or visual studio, or something.
Anyway, my work is in the Silverlight.net installation forum under a post titled "Silverlight startup error with resources - Summary of fix"
The corrected file is attached here, and I hate wasting a full day (and some of the evening) on things like this.
[EDIT] the full link of the post is http://silverlight.net/forums/p/14264/46863.aspx#46863. A distributed transaction letting me know what my link would be on both ends BEFORE i published either could have made this a bit easier.