DavidR, you wrote in a reply to my post "64 bits native soon?" in beta forum:
"Not in the initial version 5.0, but it is being developed probably 5.1.
As has been said in other topics it isn't something that will see major differences for those with x64 OS, so the priority for now is the delivery of version 5.0."
I use now Microsoft Security Essentials 64 bits, it use more momery than Avast 5 but it's native 64bits on a 64bits OS and I see the difference.