This is a very annoying VS2005 bug: PDB files residing in bin folder are sometimes added to source safe when you are doing an asp.net 2.0 project. This causes problems with source safe operations (i.e. Get Latest Version) and compile process as those files are marked read-only when they are under source control. The problem is described in details in this thread and this blog post. There is also a known KB article 915110.
One of the solutions is to download this hotfix which is rather tedious as you have to contact PSS to get it. Fortunately there is another way. You get this hotfix by downloading (public free download) "Microsoft Visual Studio 2005 - Update to Support Web Application Projects" which is some sort of addition to VS2005 (it doesn't forces you to how to do asp.net 2.0 projects in any way). I just installed it and it seems OK so far.