Original User: RockvilleGIS
I believe you only need to upgrade Server and SDE together to the same rev if they live on the same machine. I am running SDE 9.3.1 on its own machine and Server 10 on a separate machine. That compatibility is certain, but obviously I can't make use of any new features of SDE 10. Now, if you were to have SDE at 10 and then Server or Desktop at a lower rev like 9.3.1, regardless of being on different machines, that may cause some problems.
Marc