Lots of threads exist on this.
I have yet to see an article where someone has had their account canceled or been issued a much bigger than normal bill for tethering.
That doesn't mean one day they won't make an example out of the 100 biggest tetherers and send them a bill for 1000's of dollars either though. However, that has not happened since folks have been able to tether with the droid, and I don't think the droid was the first phone with tethering capabilities.
While normal usage is hard to track, if someone on a droid today was using flash, for instance, then they would know the phone was rooted.
If anyone using tethering was using their xbox to connection through the cellular network to play games there would be traffic your phone just plain couldn't make use of, and they'd know you were tethering.
It can be tracked but nobody has taken any action. Like most things in life, if you are the person tethering to torrent movies and consuming 200gb of bandwidth a month while tethered on your phone you are a far bigger target than myself who has tethered 5-6 times for a total of 3 hours to do some work at home when the power was out over the last 6 months. If they want to make an example out of someone to scare the rest, guess who they will mostly go after?
I use it if I need to, as a tool, not as recreation, and feel reasonably safe doing so.