.... Also the 4 GPU per machine limitation on Windows is now gone with the latest drivers. So you're not really losing anything by switching to Windows.
That's good news! I went ahead and gave it a shot - I received my 5850 and 5830 in the mail yesterday. CCC recognizes everything and I can tweak all five cards, but then when I fire up GUIminer, things get screwy. GUIminer can see the extra card too - but when I try to create a new miner and associate it with the fifth card - everything locks up. Tried several times.
It sounds like it might be a driver issue (like you mentioned). Any suggestions? My 5830 is sitting disconnected on the desk in front of me (and it's killing me to look at!