It already is, Bob3160 has been using avast free with windows 8 from a very early version of win8 and certainly with the current consumer preview. So it should be compatible with the RTM version of win8 soon to be released.
Though win8 comes in two versions win8 RT, which runs on systems using ARM processors (windows Slate and some tablets) and I don't believe that would work. The regular release of win8 should be fine.
