You can use <Run>Foo</Run> and place hte text stying in TextBlocks. For example:
<TextBlock>Hello <Run FontWeight="Bold">There</Run>!!!</TextBlock>
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Shawn Wildermuth
C# MVP, MCSD, Speaker and Author
Silverlight 2 and 3 Workshop
Boston, MA: April 29 - May 1
Washington, DC: June 16-18th
http://silverlight-tour.com