I'm assuming this is a Silverlight application.
It's a WPF application, sorry for not making that clearer - I do mention "WPF CheckBox" in my post, but it's easy to miss I guess :)
I've looked around a bit in the VS debugger, and I can see that the GroupBox.Header isn't part of the
VisualTree: it's a public property of the GroupBox control, inherited from
HeaderedContentControl. My understanding of how conventions work is a bit sketchy, but my guess is that the default conventions don't check for & drill into HeaderContentControl?
Perhaps I should just try and dig into the CM source, but convention handling stuff is a bit daunting :)
Thanks for your reply, I'll try poking around.