I am not convinced that URL Rewriting is really that important for a website, because 1) people will either bookmark a page or go to the root domain, and 2) search engines are really smart about handling querystring parameters these days.
However, depending on the complexity of the URLs and the number of querystring parametes, URL rewriting may serve a good role in your ASP.NET web application.
Either way, a great article on URL Rewriting in ASP.NET has recently been published on MSDN:
URL Rewriting in ASP.NET
Not only is this a great article on URL Rewriting, but a great tutorial on creating HTTP Modules. For more information on creating HTTP Modules, check out the article about Personalization in ASP.NET 1.1.