Yes, it's no longer possible. I read the rationale behind this decision on some Gnome mailing list, but I can't remember what it was.
Well. I guess I will not make that polkit GUI then
EDIT: I don't think I agree on that kind of dictation. You should be able to install ekstra administrative tools into your control center if you want.
What you want is probably a dconf-editor. You want to make a GUI to make certain changes to Gnome then that would be one way to do it. You can make it generic for all programs, or you could make it more specific to make it easier for a certain set of configurations.
One of the design principals behind dconf is that you can have multiple programs using it and multiple programs.
That is why it uses a database-type format rather then text files. There are too many race conditions you run into with multiple programs having read/write usage of a file, or series of files in a directory. One program can easily overwrite teh changes of another and corrupt a file.
Theoretically you could write a daemon that reads configurations off of a LDAP server and suck into Gnome so you can do group policies like Windows can, which I think is one of Gnome's goals.
Gnome-tweak-tool and the Gnome control panel is just two examples (just guessing, not having looked at them closely). Pessulus is probably one also, but I don't know if they have ported from gconf to dconf or what is going on with it. You can use anything you like that supports dconf. It's designed to be cross platform also.
As drag has said, lots of nice changes but nothing that fundamentally changes what was there for .0.
What I would really like to see, but the designers have shot down for some reason, is for the windows to be tagged with their icon when in overview (this was really the first extension i made in GS but due to api drift it no longer works). If you have lots of windows open on an desktop it can be hard to determine which window is which. Even apple has done this with their overview in Lion.