http://msdn2.microsoft.com/en-us/library/system.windows.controls.control.mousedoubleclick.aspx points to an event defined on Control (and thus available on Button as well) that may help you.  Also note that ClickCount may help you.

Thanks, Rob

Rob Relyea | Program Manager, WPF & Xaml Language Team
robrelyea.com | /blog | /wpf | /xaml