I found a msdn doc on recommendations for mapping datatypes between sql server 2008 and sql ce 3.5 in .NET. This actually shows a color coded matrix. Unfortunately I am
I've just been looking for this myself and came across this page: http://msdn.microsoft.com/en-us/library/ms172424(SQL.110).aspx
Looks like WP7 uses SQL CE 4.0 and we need to stick to datetime and ntext.
EDIT: I found the link above on this page: http://msdn.microsoft.com/en-us/library/hh202872(v=VS.92).aspx