As of version 12.10, Ubuntu has upgraded the version of X.org they include to the latest and unfortunately it is no longer compatible with the official ATI Catalyst drivers for some cards, specifically the HD2xxx, 3xxx and 4xxx models. The open source driver is the only officially supported alternative and, while it is fine for most uses, it doesn’t support the advanced power settings that the ATI driver does. This means that on my laptop in particular the fan runs constantly as it tries to cool down the overheating card.
So… no Ubuntu 12.10+ then?
Thankfully someone has created a PPA that successfully downgrades the version of X.org to the maximum supported version for the official ATI driver. This step is obviously quite drastic and should not be used on production systems. However from the limited time that I have been running it things seem pretty stable. The PPA (and instructions) can be found at this link: AMD Catalyst Legacy