I'll have to add in my vote for the Zenbook Prime UX32VD. I use it as my work laptop and it's awesome. Go for the UX32VD over the UX31A. The UX32VD comes with a 24gb SSD (which I use for my Ubuntu install) and a 500gb HDD (used for a Windows 7 install and the /home directory of my linux install). The benefit is that you can easily swap out the HDD for a SSD and you can also upgrade the RAM to 10gb. The UX31A and Macbook Air both have soldered, non-upgradeable RAM.

I've heard people mention the fan noise but I honestly don't notice it. The one thing I'll say isn't that great is battery life. Due to the discrete GPU, I get about 3 hours of battery life. There are some things you can do to turn off the GPU, such as installing Bumblebee.

Check this out: https://help.ubuntu.com/community/AsusZenbookPrime