Source: Running Debian Wheezy on a 2011 Air.
edit to add: Here's a quick list what running Debian on a VM gives you:
* Seamless full-screen linux with full hardware access (e.g. GPU)
* Advantage of native OS X drivers (particularly the amazing trackpad -- linux drivers crippled it in my experience)
* Instant, lag-free transition between linux/OS X -- e.g. four-finger "swipe" gesture between desktops.
* Shared linux/OS X filesystem ("shared folders")
* Zero hassle with linux wifi, custom bootloader hacks (rEFIt), etc. VirtualBox has first-class support in Debian -- everything's just an apt-get away: https://wiki.debian.org/VirtualBox
The naming of these is confusing :-)
http://www.debian.org/devel/debian-installer/
I am not sure, but you may need the additional firmware files listed on that page.
It's been too long since I used a Mac, but if someone wants to donate a recent Macbook to me I'm happy to get Debian set up and blog about it ;)
I tried both i386 and amd64 about a week ago on my 2011 Macbook Air, and only the amd64 image booted successfully.
You shouldn't need rEFIt.