Jonathan Allen wrote to me asking what order one should upgrade servers in a replication topology - pointing out that this doesn't seem obviouis from Books On Line, which I agree.
The answer is essentially the same as for any previous upgrade: Distributor first (as it has to handle new and old log record formats), then the Publisher and finally the Subscribers.
I just posted about this today as well. (http://blogs.technet.com/kevinremde/archive/2005/08/23/SQLTidbit004.aspx)
My understanding, though is that as long as you have done the distributor, it really doesn't matter which ones you do next. Publishers and Subscribers can be either 2000 or 2005.
I'm sure you're correct - I was taking a 'safe' steer from SQL 2000 BOL (mk:@MSITStore:C:\Program%20Files\Microsoft%20SQL%20Server\80\Tools\Books\instsql.chm::/in_upgrade_45ir.htm)which does give this order but does make exception - and a colleague with a priori knowledge