From what I hear, evergreen support is a priority for AMD, because it's a requirement for their fusion drivers, which they want to have ready by launch.
Work on evergreen has only recently started though, and estimates are always difficult - a few numbers have appeared here and there, but they're vastly different. Averaging those numbers, there seems to be a good chance for working acceleration in june/july, but I really doubt you'll see mainline drivers integrated into any distro yet. Go for those git repositories and have fun; preliminary bugs and unfinished features included.
I love OS drivers and all, but I'll personally stick with fglrx a while longer