We are not alone in this. slackuser46 has been asking about this at http://www.linuxquestions.org/questi...orking-671890/
The long version:
The problem is many fold. The first part is busybox 1.7.2 in slackware 12.1 and busybox 1.4.2 in slackware 12 don't support booting by label or uuid. Slackware needs at least busybox 1.10. The slackware people apparently don't know that the reason they give (they need statically linked binaries using glibc in their initrd) has been fixed. I compiled busybox 1.11.2 as a static binary and it works fine, confirming slackuser46s information.
Next the init in the initrd.gz file doesn't support booting by label or uuid. There is a very interesting article at http://lwn.net/Articles/298593/ about possibly getting everyone to use the same files. That would be a huge help.
Finally the initrd needs devmapper. I haven't figured this part out yet.