Hidden Chrome Features on CR-48
calamitylane.com
calamitylane.com
From there, you can open a terminal and start experimenting, installing outside software, and working on ChromeOS development.
...but that's pretty sweet. :)
You still need to enable developer mode to get to a "traditional" linux shell: http://www.chromium.org/poking-around-your-chrome-os-device
As other commenters have mentioned, a bunch of those work well in the Chrome Browser, too. (I'm on dev channel, 10.0.612.3)
Click to enable plugins is the killer flag in my opinion.