Please keep in mind that there could be future updates, and this is an unstable version of Gimp. By downloading and using it, you could risk losing some image files. (TRY AT YOUR OWN RISK)
This beta is only a release for Windows and Linux, you cannot use it on a mac.
Here is a forum post that gives more information about the beta:
If you run Linux, you will have to compile the source code (Or build a program from the source. This is not a .deb file, and you cannot access it through the Add/Remove applications, or the Synaptic Package manager. (This note was more for ubuntu users, sorry Fedora, OpenSUSE, etc.)
I’m not sure if this works with Vista or not, but I would assume so. Again, install at your own risk.
You do not need to remove your previous version of Gimp to install, it will replace your current version.
If you want to revert back, no big deal, just download 2.6.6 again.