@Insidious threading and core usage will remain unchanged in this initial iteration, but having the client in 64 bit puts in a better position for future optimization and memory usage.
@Vapor in the current schedule, mac users should be able to sign in, but will not be able to run the 64bit client. So you can arrive and be content, or contentedly stay away.
If and when we manage to get things compiled correctly, we’ll be rolling out the 64 Mac client as well, but there’s still ongoing work and decision making there (please see this post for more details).