As documented here, you'll need to add this:
<param name="windowless" value="bool"/>
Complete Example:
<object data="data:application/x-silverlight-2," type="application/x-silverlight-2" width="100%" height="100%">
<param name="source" value="ClientBin/TestSilverlightStuff.xap"/>
<param name="onError" value="onSilverlightError" />
<param name="background" value="white" />
<param name="windowless" value="bool"/>
<param name="minRuntimeVersion" value="4.0.50401.0" />
<param name="autoUpgrade" value="true" />
<a href="http://go.microsoft.com/fwlink/?LinkID=149156&v=4.0.50401.0" style="text-decoration:none">
<img src="http://go.microsoft.com/fwlink/?LinkId=161376" alt="Get Microsoft Silverlight" style="border-style:none"/>
</a>
</object>