A number of *.browser files are shipped with .NET:
C:\Windows\Microsoft.NET\Framework\v2.0.50727\CONFIG\Browsers
The runtime uses regular expressions from the *.browser files to match against the incoming User-Agent string, and then sets a bunch of properties based on each match it finds (there can be several in the hierarchy).
If you need in-depth mobile device support, consider installing the MDBF, which adds support for about 400 devices:
http://mdbf.codeplex.com/