How does IsMobileDevice work?

MSDN makes it sound so easy to detect a mobile browser:

if (Request.Browser["IsMobileDevice"] == "true" ) 
{
    Response.Redirect("MobileDefault.aspx");
}

Actually, it looks like you can also just check Request.Browser.IsMobileDevice. But how does this actually work? I don't even have a .browser file... what's going on behind the scenes here? Are there some built-in defaults for ASP.NET 2.0?

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/

Now, after 4 years, it's even more simple

Request.Browser.IsMobileDevice