So is the MBA totally unsuitable for running Ubuntu (e.g. wireless won't work), or just a bit more of a hassle to get working properly?
Every part in the MBA has Linux drivers, which isn't surprising in any way, since there are no magical parts made by unicorns in there, just the typical combination of generic PC components you will find in other laptops. What sets the MBA apart is the way the body is constructed and how it holds all the stuff inside together in such a small enclosure.
MBA is far from ideal Linux machine - Apple's EFI implementation is well known to be crappy to the point Linux kernel developer mjg59 routinely blogs about it. Power management sucks - for example - it runs hotter, battery dies in 2-3 hours tops. On my X220 I can get 6+ hours and it never gets hot.
[EDIT]: Check this out http://ubuntuforums.org/showthread.php?t=1810275 for the pain involved. In comparison my X220 had one minor issue that occurs under heavy load but can be easily fixed.
I'm posting this from my MacBook, but I'd say only one thing: Focus follows mouse. That's the one true way.
Matter of personal preference really. My "Desktop" machine is Windows 7 which happens to work better for my needs than OS X - Blu-Ray, AnyDVD, Hulu/Netflix without fan noise etc. That's where I do all my media/email/browsing/chat etc. Linux is primarily a development machine - I find it better for OSS development than OS X. (Plus in my case I hack on the kernel some times so that's another compelling reason.)