How obvious is this? Package up a 250GB hard disk, a four-port USB hub, and a USB Ethernet adapter into a nice, small package (something like the latest Iomega “Silver Series”, slim enough to stick in your pocket – about 5″ x 3″ x 0.5″ and 7 oz.), with a captive USB cable designed to slot into the MacBook Air. External power option, in case we want to coddle the MBA’s battery. Give it a MBA-style brushed metal finish, and price it at $200. Who’s going to do it first? ((This neat unit is awesome, but a bit too big.))
Of course we still need Apple to fix iTunes, so that you can sync a subset of your full library onto a MBA, just like you can with an iPod or iPhone.