Setting up your own authentication method for SSRS has been discussed before (a lot!) but I've never seen an implementation of it that I could understand because it requires a fairly high-level of programming knowledge.

There are two places I'd direct you aside from a Google search. One is the book Applied Reporting Services as I'm fairly certain it directly addressed it (check the table of contents).

Secondly there was a sample in the Reporting Services 2008 Samples. It shows you how to have a web page with its own credential system, and you could link it to the SSRS back-end to check those credentials. I take it that's what you want to achieve.

Please post back with your findings. I've always found it interesting.