avast! 5.0 (even 4.x) supports 64bit OSes already.
Version 5.1 will support the boot-time scan in 64bit OSes. Whether there will be a fully 64bit version of 5.1 or not, it wasn't decided yet. From users' point of view, it doesn't really matter - the "hybrid" version (64bit drivers and a few DLLs, the rest in 32bits) works just fine.