From what I understand, you can do so if you purchased a model that is capable of accessing mobile networks (WWAN 3G models). For more info you can visit Google's Chromebook support page that tells how to activate a Sims card: http://support.google.com/chromeos/bin/answer.py?hl=en&answer=1095357
I haven't found the answer to that yet, but what I can tell you is this thing is ingenious. idk if you have a main computer, but this is virtually an extension of that. Yes you can use the web browsing fine and dandy, but the features of the cloud are crazy, I can't fully comprehend it. But what I do know is, by accessing the remote desktop with my desktop pc, you can virtually control your computer from anywhere you get a connection. So... virtually, ALMOST anything you can do on your home computer, i.e. itunes, wmedia player, microsoft word, yada yada yada, you can still do on this computer.
You are speaking about buying a 3G model. If you are going to do that, then I recommend getting an actual laptop as this chromebook has limited capabilities. There is no Android here, and the chromebook is for people who are not heavy users for surfing the web. I don't think there is Flash in here, buy a laptop that has more capabilities and also bigger.