Had the same problem with that mysterious lump of "others" memory, I had like 40gigs of what-and-where-the-hell-is-this.
It turned out to be my iTouch 4 (30gig backup) and iPhone (10gig backup). I deleted them from my drives and BAM, my Air is 40gigs lighter
/library/Application Support/MobileSync/backups
Is there a way to configure the location where iTunes sends these backups? I have an iPhone and an iPad, and definitely would like to back them up, I just don't want the backups eating up all the space on the small SSD in my MBAir. I have a 1TB drive that's connected to the Air when it's on my desktop, I'd love the backups to go there. Possible to configure that?