I'm interested in the answer to this same question.
A number of posts seem to imply that people are doing independent
installation of Project Pro SP1 and Project Server SP1.
However, an MS Tech I was speaking with (on another support issue with
MS Project 2003) stated very emphatically that SP1 *must* be installed
on the clients and server(s) at the same time.
The supporting docs for SP1 don't mention this. Given the difficulty
it would cause us - a small shop - to do the server and client
upgrades in sync, I can't imagine a company with hundreds of users
pulling this off easily.
That said, I installed Project Pro 2003 SP1 on a test client and was
able to hit our non-SP1 server with no apparent problems. I didn't
bang on it too much, though.
-jmk