I came across an interesting forum post by Suzanne Cook today.  Here's what she had to say about 1.1 assemblies and 2.0 assemblies in the same process:

As of today, only one version of the runtime is loaded per process, no matter which versions of assemblies are later requested.  So, if the v2 CLR is already loaded, it will successfully load a v1 assembly and run it against the v2 CLR (not the v1 CLR) without reloading the already-loaded v2 CLR .dlls.

While we're on this topic, you may want to check out http://blogs.msdn.com/suzcook/archive/2005/01/26/361092.aspx for related compatibility information.