Often I get mails from people saying that the Mobile Internet Toolkit from Microsoft which they are using to build mobile web applications is not targetting a specific phone. My first piece of advice is to get the latest ASP.Net Mobile Controls Device Updates. This is available at http://msdn.microsoft.com/mobility/othertech/asp.netmc/mobileweb/aspmobiledrivers/default.aspx
Microsoft constantly updates the list of mobile device information in these updates. The Tested Devices page lists the various devices which one can target with the ASP.Net Mobile controls. Besides this if one wants to target a specifc device one needs to look at the Extensibility Developer's Guide.
Device updates extend Web applications to work with additional devices and browsers. When you install a device update, the <browserCaps> section of the Machine.config file is revised to include additional entries for the new browsers.
ASP.Net 2.0 changes this, no longer is Machine.Config altered but there are .browser files which contain browser information.