Disclaimer #1: This was smoke-screen tested only so it will have issues. If you find them then please post them in the comments on the blog to help others.
Discalimer #2: Don't do this on your main development box unless you holiday here a lot, and definitely don't do this on a production box (unless you watch re-runs of this and are wanting to participate).
There, now I have got that out of the way - you are still keen to play with generics and all the crazy cool stuff in VS.NET 2005 beta 1 and at the same time you have production work to do in VS.NET 2003 with BizTalk Server 2004 and you want them to work all on the same machine? Ok, seems reasonable to me. The main issue is that you need to create a set of .config files to ensure that the BizTalk Server exe's and services run using the 1.1 version of the framework regardless of the fact you now have 2.0 installs. The way to do that is to edit or create config files for all these artifacts. Of course that would be tedious so here is a batch file and all the config files you need (hopefully).
One generic issue we are aware of that “debug attach to process” doesn't appear to work for VS.NET 2003 .NET CLR 1.1 applications in general post install. This makes it pretty challenging to debug custom components inside Orchestration.
Thanks to Aleksey and Andy Reeves for the information and pre-packaged configuration files.