As Windows Phone 7 is basically just Silverlight 3 I suspect that it will be fine.
There are multiple posts about using NUnit with Silverlight, so give them a try and you will probably have lots of success.
Here is one on using TestDriven.Net and NUnit with Silverlight: http://weblogs.asp.net/nunitaddin/archive/2008/05/01/silverlight-nunit-projects.aspx