I don't know if this could work for you but my solution (with a 330GB itunes library) is to create 2 libraries for my MBA.
The first library is small and just includes some of my top rated stuff. That is on my local MBA drive. If I'm remote and just want to use my MBA, I can still get my favorite music, a couple movies and a bunch of TV shows. I also have a second, much larger music collection on a portable WD Passport 320GB drive. If I'm in a hotel or something and want to pull up more music, TV or movies, I plug it in and go from there. Finally, if I want the full fledged experience, I guess I could back to my mac to access to full 330GB library on the desktop mac.
These WD passport drives are a real nice addition to the MBA. With it, I only add a tiny bit of weight and very small (physically) device, but it allows me to get 5 times as much storage as just the laptop alone. Great for traveling if you need a bit more space.