You get a CR-48 in the mail. You want to use it but the wireless network only allows access to specified MAC addresses. How do you determine the MAC address?
Look for 'ifconfig' and select expand. You'll see 'wlan0'. 'HWaddr' is the MAC address.
In what is probably a more recent development since you originally asked this question, you can see the MAC from the settings pop-up in the lower right corner. Click the clock, then on "Connected to [SSID]". In the lowest right-hand corner of that popup is an "i" button that looks like a stop sign with the hover text "Network Info". Clicking on it reveals the IP and MAC.
Since ChromeOS 33 (possibly earlier) you go to
chrome://settings, click the Advanced Settings link at the bottom, then click through Network Proxy Settings to bring up the dialog box. The MAC address is in there, possibly behind a tab.
This link is not Chrome OS specific but the general process is similar.
On my Samsung ARM Chromebook (1st gen) I can click the little Wi-Fi icon in the corner, select the network I'm currently on, and then click the "network" tab, which takes me to the same place.