Just installed slackware (slamd64) 12.2 on a Mac Pro. Once I figured out the steps, they were surprisingly few. Permalink to the following information (but probably more updated, with better layout):
slackware (slamd64) on OS X
* how to install slackware (slamd64) on a mac pro
* "bootcamp" is not used
* starting from unformatted disk
* during install of OS X, resize the OS X partition down so that there is space remaining for additional partitions. In my case, I gave OS X 20GB to install in.
* Install OS X
* note that OS X installs an additional 210MB partition which it uses to extend the EFI firmware. We do not touch it during this process.
* install refit (only really needed for gptsync)
* reboot from slackware (slamd64) DVD
* use parted to create an ext2 partition for the OS, and a swap partition.
* reboot. From refit boot menu, choose "partitioning tool", which will automatically synchronize MBR with the EFI partition information. Note that there are probably other ways to synchronize MBR and EFI, perhaps (diskutil?)
* reboot from slackware (slamd64) installation media. From this point, slackware (slamd64) should install as normal.