is your credit history ruined, or merely dinged? Is the blow
Any bad credit rating event is recoverable given enough time / money to solve the problem. As far as "Ruined" vs" "Dinged", well, that's a matter of opinion; some people think that one bad item is the end of the world, others not so much. You will have an unpaid debt listed on your report. This will drop your score. The amount it impacts the score will depend on other factors in your report.
Can the carrier try to get the money back in court? I assume you'll
wind up dealing with a debt collector.
Yes they could go to court, but that's unlikely at least in the short term. Far more likely is that the debt ends up sold to a debt collection agency for pennies on the dollar. The debt collection agency will harass you until you pay and they might file in court if they think the debt is more than enough to cover the court costs.
Will this affect any other relationships you have?
Possibly. A bad rating may make it more difficult to get credit in the future. However that depends on numerous other factors such as your entire history. It could even prevent you from being hired from certain jobs - not many of them, but some.
Is it criminal?
Read this: http://www.startribune.com/investigators/95692619.html The US does NOT have a debtors prison. However if the company decides to file a court case and you fail to appear or fail to abide by the court ruling then, in some states, you could be committing a crime and may be thrown in jail. At which point you are on the hook not just for the original fee but potentially a plethora of other costs. Never mind the loss of reputation when your friends, family and coworkers find out that you are sitting in jail.
At the end of the day, just pay the debt. If you agreed to the plan and the plan has an early cancellation fee then the moral and ethical thing to do is pay it. Trying to see how bad it would be to ignore it isn't the right way to live.