If the person who answered this question is right, you cannot put a binding as the value in a setter in a style in Silverlight. Which is a shame, because I have 4 textblocks
I had the exact same problem a few days ago and found the following blog post.
http://blogs.msdn.com/b/delay/archive/2009/11/02/as-the-platform-evolves-so-do-the-workarounds-better-settervaluebindinghelper-makes-silverlight-setters-better-er.aspx