I'm so glad my company is paying the huge wad it costs each year to subscribe me to MSDN - so that Microsoft can ignore bugs in their product altogether:
* Microsoft Connect: Expression Blend 3 ignores and hides my attached properties used in styles
* social.msdn.microsoft.com: Expression Blend 3 ignores and hides my attached properties used in styles
Reported on August 6th 2009 (about a week after Expression 3 release). Response: Nada.
Sure - creating customizable buttons for WPF is such an unimportant feature... sure.