ASP.NET 4.0 makes it easy to implement clean, SEO friendly, URLs using both ASP.NET MVC and now ASP.NET Web Forms (you can also have applications that mix the two). 

The URL routing engine makes it easy to register URLs of any shape or format and map them to any handler you want.  Because the URL routing engine can be used for both mapping incoming URLs as well as generating outgoing URLs, you can at a later point change the URL mappings  and not have to modify any page or controller specific code to reflect them – which makes building SEO optimized applications much easier.

Hope this helps,

Scott