I don't have the 5.0 installed or in the GAC. I even deleted the files from the network directory where we house stored dll's. The project is a web site type instead of a web project... so the references are marked auto update unless I delete
the .refresh files. I've deleted those and still get the overwrite.
It might be worth mentioning that the overwrite happens almost right away after I hit run or build.
I did update a WCF service recently to 5.0. However, I did it manually by including the dll's and editing the web.config. I deleted all references to that service once I started getting the errors caused by the automatic dll upgrade, though.