I posted an answer over to the similar question:

Is there any way to set a MacBook Pro to not sleep when you close the lid?

that applies to this question as well. Since I found this question in a search, others will too, so go there for the discussion.

How can I keep a macbook running when the display is closed?
I use Macs, Ubuntu, FreeBSD, OpenBSD, and Fedora for different purposes. I'm fairly busy and trying out a new distro takes a lot of time, but I hear lots of good things about Arch Linux from people I admire. I do mostly scientific computing (and some web development), and use Linux as both a desktop and a server. Does Arch offer anything I'm not already getting from one of my current installs? I'm particularly interested in the differences between Arch and Debian
