We'll I thought at first this was some kind of beginner question, but I see
you want "the real thing!!", so here I come!!

Microsoft has it's own implementation of the Device Capabilities assessment
written in the ASP.NET Engine (HTTP Pipelines) this pipelines are a
collection of HTTP modules

HTTP modules are classes that have access to the incoming request. These
modules can inspect the incoming request and make decisions that affect the
internal flow of the request. After passing through the specified HTTP
modules, the request reaches an HTTP handler, whose job it is to generate
the output that will be sent back to the requesting browser. all the
information about the mobile device capabilities is written in the request.

In fact, you can see a precise list of what modules are used by default by
going to the machine.config file (located in the
$WINDOWS$\Microsoft.NET\Framework\$VERSION$\CONFIG directory). Fist you can
search for an element called <sectionGroup name="system.web"> and then the
section corresponding to the devicefilters (<section name="deviceFilters"
type="System.Web.Mobile.......). And the mobile controls (<section
name="mobileControls" type="System.Web.UI.MobileControls...) they are the
main classes for mobile web development in asp.net and as you can see they
are in charged of handling requests from mobile devices.

You'll also find a <mobileControls> tag and after this tag you'll locate al
the device filters and if you have installed device updates you'll see the
changes made by them in this section. This is the most interesting part
because you'll meet the actual classes who take care of the rendering, like
System.Web.UI.MobileControls.Adapters.WmlTextBoxAdapter capable of rendering
textboxes to WML and a whole bunch of them for all the devices supported by
the framework.

then there's the <browserCaps> tag where you'll find all the set of
capabilities ASP.NET defines for each type of browser it supports, as you'll
see it's some what similar to WURFL and also very plain and simple.

You can keep browsing the machine.config and the whole config folder and at
the end you'll get the picture of how all the dirty work is done!!!!

Hope its all clear now. Cheers!!!
--

Juan David Gomez A.
Microsoft Certified Professional
Analista de Desarrollo - PSL S.A.
Web and Wireless Banking
Medellin - Colombia