I am currently in my first clinical rotation, and just spent a day with an ER PT last week. I am at a large county metropolitan hospital that has a formal ER PT program. There are 3 PT's that work in this ER, and 4 PT's that work at another ER in the city. Both are Level 1 Trauma ER's. I must say that it was a fantastic experience. Like all have said, it is not like you are running in with the MDs and nurses when a GSW patient rolls in. It is actually more closely aligned with outpatient practice than inpatient in terms of how you bill for treatment, and use of diagnostic skills.
Apparently, formal ER PT programs are pretty rare with only 20-30 formal programs in the entire country, and usually require a good deal of experience in multiple areas (ortho, neuro, etc.). The PT I was with has been practicing as an ER PT for 10 years, and is involved in promoting and sort of advocating for formal ER PT programs, and has compiled data and done research that he presented at CSM this year about the benefits of ER PT treatment. It was interesting stuff. (Sorry I don't have a link or copy of the diagrams!)