We have a massive client facing web application here. We recently re-wrote the buy online experiance to make it quicker. My suggestion of using ASP.Net-MVC was not taken o
Chapter 12 in Professional ASP.NET MVC 1.0 is called Best of Both Worlds: Web Forms and MVC Together. It covers exactly the issue you raise. The good news is that there is not a lot to it. The book was written by team rock-stars Rob Conery, Scott Hanselman, Phil Haack, and Scott Guthrie. I found it instrumental when learning ASP.NET MVC.
Also, here's an article called Integrating ASP.Net MVC Into An Existing ASP.Net Web Application that discusses it.
UPDATE: Turns out that you can download a copy of the draft Best of Both Worlds chapter for free. This is an "Uncorrected Draft - Not Final Book Chapter" but might be worth checking out.