I have a wifi only Acer chromebook. I also wanted an option of a wired connection when I go visit my sister who doesn't have a wifi router. I bought the "Trendnet TU2-ET 100" USB to 10/100Mbps Adapter and it works, I am using it now as I type this message. It works right out of the box, you can ignore the included setup DVD. I got it from Amazon for $17.69.
Google's help on this says a wired (USB) connection take priority over wireless and it seems to work that way. Instead of wifi bars in the upper right corner of the screen I have an icon that looks like an Ethernet port.
That's good to know! I was testing my chromebook with the wired tethering app on my phone the other day and it works too. just plug it in and go, but always check with your carrier about their policies on tethering.