I was able to get RC1 installed beside 2003 ok.  The only problem I can remember was VSS.  My 2003 install seemed to break, with regards to VSS 6.0.  There was another case where installing 2005 beside 2003 broke everything in 2003 (wouldn't create web apps, couldn't debug existing ASP.NET projects, etc).

I'm wondering if anyone is far enough that they've downloaded and installed RTM 2005 beside 2003.