We have implemented Single Log On using a customized version of Forms Authentication so we can log in with one site and jump to another site (both in the same domain, e.g. *.mycompany.com). We did this two years ago. We have six sites total this works with and they were all running ASP.NET 3.5. Some of the sites however are classic ASP that run in Integrated mode so we can take advantage of Forms Authentication and role authorization. It's worked fine for us. We just need to ensure that the machine key and Forms Authentication settings are the same across all six applications.
We have recently needed to upgrade one of our classic ASP apps to use ASP.NET 4.0 (using an HTTP Handler for retrieving images) because one of the DLLs we need to use was build using .NET 4.0. We changed the project to use ASP.NET 4.0 and made the necessary web.config changes, and changed the app pool in IIS to use ASP.NET 4.0. We do our development on Windows ...
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