I have at various times installed various Ubuntu-based distributions on my dual boot (rEFIt'd) MacBook. Linux Mint 11 worked the best out of the box, for me, and until about a week ago was my primary OS: trackpad, ACPI, 'hibernate' super-function keys, all worked without additional effort.
Last week I installed LMDE (Mint Debian) xfce on a third boot partition, and with some effort have almost everything working (isight's still down) -- hoping to make it my primary OS (a nice scaled-back rolling distro of Debian testing along the lines of #!).
That said, I've been doing the same research. So far I've only read good things about ArchLinux on MacBook, and that's my side-project this A.M. (bye-bye Mint 11). Anyone willing to install ArchLinux already expects to assemble their own perfect distro, so out of the box functionality isn't expected. Arch DOES have by FAR the best community support, and comprehensive guides to MacBook/Pro installs.
Another answer on here mentions that battery life seems shorter on Linux than Mac OS X. That's true -- plus they run hot. Again, ArchLinux community seems to have all the answers, even if you're installing a different distro, check out their online "MacBook Pro Arch Wiki" page for advanced MacBook tips.