I can't speak 100%, but in my experience, they aren't even backwards compatible within the same service pack/revision. When we upgraded our test platform from 10.0b to 10.1b, I had to install both clients on any system that needed access (including ODBC access) to both live and test--the 10.1b driver would not connect at all to the 10.0b database, and vice versa.
But to make things even more difficult, our ERP vendor specifically said to take the server to 10.1b sp1, but the clients could be 10.1b sp1 *or later*. So I left the server at sp1 and the clients I took up to sp3. It crashed our database a couple of times. I was told that the documentation should have said 10.1b sp1 or later for the database as well.
I don't often complain about Progress, as I think many issues are actually the result of third parties (ie, ERP vendors) that write crap--but the fact that using an SP3 driver against an SP1 database would crash the database is just plain bad.
