I am running GUS on 1.4b without problem on 2 sites. I did notice something yesterday that the IP filtering doesn't seem to work. But, I haven't looked at it carefully enough to say that's the problem with 1.4b and GUS.
You don't have to turn register_globals off to run 1.4b. It is just a good news for the near future. For the time being we might have to leave it on until all the plugins are made for register_globals off. Of course, if security is your top concern, don't take my advice. I run my sites on my own server and not a share hosting, so having register_globals on doesn't risk other people's security, but my own.
Sam