$30 for the ThunderBolt GigE adapter. It's possible people will lose them but you do have to physically disconnect the RJ45 from it before you can leave (unless you have a very long cable) so it's not a big problem really. Easier to lose your untethered car keys, wallet, etc.
Yeah, at $30 I'm not as upset. I still predict they'll be annoying, but if you bring your own cat5e cable too (vs. plugging into an existing cable), it's less likely it will be left behind. (I'll probably order one for my 2011 MBA anyway).
Keys/wallet/etc. remain in pockets when not in use, though -- so they're a LOT less likely to be abandoned than something which is small, connected to another long cable, etc.
