I get an access denied error when I try to attach to w3wp.exe from Visual Studio 2008 on Server 2008 64-bit, even though I have the application pool set to run as my own unprivileged user. On Server 2003 I was able to do this.
Something weird is going on because when I run Process Explorer as myself I can't see any detailed process information about the app pool (as if I didn't have permissions for it), but when I run Process Explorer as local administrator I can see that it is in fact running as my user. w3wp.exe even shows up in the list of processes to attach to in VS.net without checking the "show processes from all users" box. But the "running as" column is blank and I can't actually attach.
Right now I'm working around it by running msvsmon.exe as administrator and attaching through that (after giving permissions on it to my unprivileged user). But I shouldn't have to because it's my process.