Got a question on this today during my talk so referencing a couple of articles here for those interested.

• SD Cards in Windows Phone 7
  http://blogs.msdn.com/b/dcook/archive/2010/11/08/sd-cards-in-windows-phone-7.aspx
  
• Windows Phone 7 Secure Digital Card Limitations
  http://support.microsoft.com/kb/2450831

Joel Reyes