I was just installing Mac OS X Lion 10.7.2 last night and faced an issue on my Mac Book Air: lack of disk space. Then I not only tried uninstalling some useless apps I have installed, but I also moved my iPad backup space to an external hard drive which I formatted last night for my Mac usage. Here is how I did that:
chflags nohidden ~/Library/
Quit iTunes
Copy your ~/Library/Application Support/MobileSync/Backup folder to your external drive (the ~ means “home directory”)
Delete the original “Backup” folder by dragging it to the Trash
ln -s “/Volumes/My Stuff/Backup” “~/Library/Application Support/MobileSync/Backup”