This sort of things happens when an upgrade has a bug, but this time it appears that there is just additional functionality rather than any problems. The upgrade applies to Radeon HD 2000, 3000, 4000, 5000, and 6000 series graphics cards, and Radeon 3000 and 4000 series chipsets.
AMD made a couple of enhancements to its HydraGrid feature found in HydraVision, which is now aware of user created Bezels with Catalyst 11.5 installed, and also includes an 'Automatic Grid' option that aligns to the user defined Eyefinity display layout.
The site seems to suggest that the upgrade is more about fixing bugs in the driver's operations with Windows 7, and one in Windows XP. Nothing major anyway.
http://support.amd.com/us/kbarticles/Pages/AMDCatalyst115ReleaseNotes.aspx