A few days ago I mentioned that my colleagues and I worked out some kinks and got SQL Server Reporting Services (SSRS) to use Forms Authentication and integrate withour ASP.NET project. It was not as straight forward as we had all thought heading into this endeavor and we did not find much help on the web. But the help we did find was very useful. For example, here is a very good article posted on MSDN that discusses and demosntrates how to implement Forms Auth with SSRS. The article discusses what needs to be done to set up a sample application (which is downloadable) to work with Forms Authentication. We still had to tweak it here and there to meet our needs of course, but overall this is the scheme that we followed.
I'll followup with other tidbits of our findings, but if you are looking for a good jumpstart into SSRS and Forms Auth, check out this article.