Recently SMP support was added to the rt2500 CVS and thus as it is such a common device, I've added packages for beyond and vanilla, to [unstable].
rt2x00 has been approaching a somewhat workable state, and it can be used by some. It doesnt work for me, but feel free to give it a shot, it's also in unstable. I'll try and update this as often as I can. if you decide to build the PKGBUILD yourself, be aware that it will complain, as the directory inside the tarball keeps changing names. You need to update this to reflect the latest snapshot. I have not used public CVS as the rt2x00 project has been having problems with it being out of date.
To use rt2x00, take a look at the wiki wiki.archlinux.org/index.php/Rt2x00