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 believe the page you're after is this one. SQL CE 3.5 is the same no matter what platform you're on, so the "WP7" part of the question doesn't matter.
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