Search around a bit through the user sites / blogs, but as I recall, distributor is the critical component here to have as 2005. Someone out there has a great website regarding SQL replication, and I've lost it.
If I currently have a publisher and distributor on 2000, and want to upgrade subscribers to 2005... it is necessary for distributor to be of equal or greater versions that subscribers. We are working on this ourselves. In our case, we have test enviroments to set up play senarios.
Several of our subscribers are planning on upgrading to 2005 before our publisher/distributor combo. I'm considering separating distributor as 2005 and from what I read so far, this will work.
Sorry if not accuate, as I can't reference anything, but I think/hope so
MCITP - Database Administrator (SQL 2008)
MCSE: Data Platform (SQL 2012)