I spent a fair while with the latest Ubuntu, 6.06, on my iMac over the weekend. I really wanted Ubuntu to be “better” than OS X for my purposes, but ultimately it wasn't.
What I got was a system that would give me 6 text-mode consoles (yay!), Xorg with GNOME (took a while to get 1440×900 resolution though) on VT7, and Mac on Linux on the next screen, VT8. Wonderful!
However, I'm hamstrung by my hardware – there does not appear to be any 3D driver for the nVIDIA GeForce4 video card in the machine (Intel machines have the choice of using a binary-only driver), and the MoL console wouldn't get over 256 colours. OS X isn't useable in this state.
So for the time being I'll stick to OS X to keep the other user of my computer happy :-)