Looks like it will be available whenever 3.0 comes out. You can download the nightlies in the meantime to try it out. I know other methods have been available to get the functionality in FF, but the more people that have it automatically enabled, greater likelihood developers will use it. Of course, then I'm sure we'll find all sorts of flaws in the actual implementations, but it's still a good idea in general.
I blogged about it with a few more details: http://www.networkcomputing.com/blog/dailyblog/archives/2007/03/no_cookie_for_y.html
Original bugzilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=178993
