From what I've seen in terms of the Chromium OS source code, its Gentoo linux is very stripped down. Just because something works in ubuntu does not mean it will work here.
In the case of 3G USB keys, a vital component called usb-modeswitch is what is needed to get 95% of the USB keys to switch from cd-rom (windows and mac drivers) mode into modem mode. While this program is found on Ubuntu it is not emulated by Chrome OS's flimflam program, yet.
Anyway, I expect it to get better, but for right now it's not available just because of the wide range of USB key hardware out there