We've had a USB to Ethernet adaptor since my wife got her 1st gen MBA in 2009.
You haven't had a Thunderbolt to Ethernet adapter, and a USB to Ethernet adapter that predates USB 3 isn't going to do you much good for gigabit networking.
Furthermore, you'd better be able to plug things into those two ports, because without those ports to provide bandwidth and connections, the machine's a little light on functional ports.
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Well, I guess you could always just buy the $999 "adapter" from Apple called a 27" LED Thunderbolt Display. It has a FW800 and Ethernet port built-in... and you can walk into any Apple store and pick one of those bad boys up.... TODAY. </sarcasm>
Woah. I hadn't noticed that. That is really, really, tempting.
Only problem is, I use a KVM switch and three or four computers, and I don't think I could just ditch my current setup.
Seriously, though, if that thing had a DVI input too, I would probably get one and use it and be happy. That would be Good Enough for my actual application. ... wait, no, I think I'd need one with VESA mounting.
EDIT: They have a VESA kit for that. Now if only they'd done a new Mini, that would seriously solve a number of my problems. Or a Mac Pro with Thunderbolt, but what kind of idiot would want the highest-performance port adapter on the highest-performance machine? That's crazy talk.