Scott Hanselman did a podcast on a module device database that Microsoft made available on Codeplex. It's a .browser file that you put in your ASP.NET (version 2.0+) website, which then helps the framework define the Request.Browser property more closely. You should be able to get a lot of the information that you need from there.
However, that project is no longer supported, and when you're dealing with user agent detection, having an up-to-date resource is very important. You should probably use a similar project, such as 51degrees.mobi or WURFL.
Even without that, at the least you can check Request.Browser.IsMobileDevice.