I've been using Slackware 12.1 on my 900A Eee PC for a while now. Before that, I was using it on a 701. In both cases, it worked really well and was very responsive (I chose to use Xfce as my desktop environment since I wasn't that familiar with Fluxbox at the time). I've even done some Common Lisp programming on them with a locally compiled SBCL. One thing I noticed in particular was that I got more free space out of my 4gb SSD than with Ubuntu-Eee or the default Xandros-based distro.
Slackware 12.2 should actually work out of the box. 12.1 required a modified kernel and a few other things (packages at http://slackeee.strangled.net/
). The only thing to keep in mind is that I do a lot of things by hand on my boxen, such as set up wifi connections on the command line, so Slackware may not be the best choice for you if you're not used to this sort of thing.
I did a short writeup on installing onto my 701 on my blog, as well: http://blog.partition36.com/2008/07/04/eee-pc-meet-slackware/