I already had Ubuntu installed, but I wanted to install Slackware on a spare partition and dual boot. On my first attempt, I just went for it and installed Slackware, LILO and all. That screwed up my Ubuntu installation, and I couldn't quickly make LILO boot my Ubuntu installation.
I ended up reinstalling Ubuntu. On my second attempt installing Slackware, I decided to install Slackware, but skip the LILO installation, and research how to make GRUB boot slackware.
It turned out to be fairly simple to do this. Here's the entry from /boot/grub/menu.lst
title Slackware Linux (Slackware 12.0.0) (on /dev/sda5)
kernel /boot/vmlinuz root=/dev/sda5 ro
The main thing is to get the right partition for root. The partition numbering starts from 0, not 1. Extended partitions start with 4 no matter what. So, as you can see, root is on my first hard drive, on the first extended partition. It also wants the read only (ro) flag to be set on bootup. Otherwise it will complain. Other than that, it's a snap.