Got my RPi on 2012-April-19. Currently running the customized Debian image, since I know a little more about Debian than the other supported distros (Arch, Fedora).
One of my project ideas will be to get Slackware ARM running on the RPi. I would think that the Debian kernel and modules, with Slackware ARM userspace, could be hacked together to work, but that's only an uneducated guess. Slackware ARM currently has three installer images, for the three supported platforms; I eagerly await the fourth, Raspberry Pi.
Status update 2012-05-02, late evening: I have a Slackware ARM setup that crashes in /etc/rc.d/rc.S. But at least that means /sbin/init is running from the flash card. Yay!