I've been working on a project at work which involves hosting PHP pages in a IIS 6.0 server. The site uses querystring parameters to pass to single PHP page which in turn calls the requested include pages. The site works well but Google has difficulty crawling the site because none of the pages use "friendly-URLs". We found some third-party products to install on IIS like http://www.micronovae.com/ModRewrite/ModRewrite.html but there they're a pain to get working and cost $$.
I started looking more into URLRewritting and was happy to see that this is built into the 2.0 Framework. Here's a nice overview of it from Scott Mitchell:
http://aspnet.4guysfromrolla.com/articles/011007-1.aspx