As Chris Cavanagh helpfully points out in WPF – Easy rounded corners for anything, you can add a corner to anything in WPF & Silverlight.
I just pinched a little bit of it to use:
<Grid> <Border Name="mask" Background="White" CornerRadius="7"/> <StackPanel> ...
..but that seems to work alright for the moment.
It’ll be nice when I’ve managed to get a good handle on this; Microsoft Blend really isn’t as bad a program as it could be afterall.