In this post, Mike Ormond discusses accessing
XNA APIs from a
Windows Phone 7 Silverlight application and touch gesture support.
Source: Mike Ormond's Blog
Although Silverlight offers manipulation support, it has no direct support for the higher level concept of “gestures” (eg tap, flick). You could certainly build that yourself in Silverlight OR you may be able to use the gesture support from the XNA Framework. I say “may” as I haven’t tested this exhaustively but my initial efforts suggest it seems to work just fine.