My Macbook Air is the first edition - 1.6 GHz, 80GB HD (not SSD)
Insert the Ubuntu CD (I have the Superdrive), reboot, hold C to boot from CD and when Ubuntu live comes double click the Install icon.
Follow the on screen instructions.
For simplicity I decided to nuke OS X and let Ubuntu have the entire HD.
After the installation restart and everything works out of the box.
I even installed Skype and the iSight camera works great.
What an awesome job the grub guys have done supporting EFI too!
UPDATE Microphone is not working and apparently has never worked - https://bugs.launchpad.net/mactel-support/+bug/268301