Please remember that this 64-bit client is still in beta, even though you can play the live (non-PTR) game with it. This means that you might experience unexpected crashes and other bugs. Be sure that if you do experience them, you report them right away so Blizzard can fix everything up.
I've installed this live 64-bit client; however, there still seems to be a version validation error with the servers. This is probably a quick fix that Blizzard can take care of, and we're hopeful that everything will be solved soon.
Download the Windows client here and the Mac client here. The full blue post after the break.
Update 3:10 p.m. EST Blizzard pulled the download information from its support forums and posted a message saying that final preparations were being made and the client would be up soon. We're assuming this is due to the validation errors with the servers. We'll update this post when we know more. The download links and everything still work, however.
Update 4:05 p.m. EST Blizzard posted an update this afternoon:
Update 6:20 p.m. EST The 64-bit client will not be usable today, due to unforseen issues. We're leaving the original blue post with the download links up after the break.
Update 2:40 p.m. EST on 2/1/12 The 64-bit client is now functional.