I saw a post in one of the newsgroups today that referenced a piece of information Microsoft published on how the Optimizer makes decisions about remoting certain operations.
It's slightly out-of-date, and I'll work on trying to get it updated. Specifically, SQL 2005 will remote uniqueidentifiers (and it will support statistics over them as well). We'll remote queries from SQL 2005 to SQL 2000 that contain filters comparing columns to constant uniqueidentifier values as well.
We published a research paper last year on how the Distributed Query feature works in more detail. While it does not cover every practical detail of the implementation, you may find it as an interesting reference if you use distributed queries.
If you have other remoting questions/problems, please post comments on them and we'll see if we can get them answered for you.