Florida,
I'm not in residency yet, just finishing up second year (finished classes Friday!!!). Incidentally there's a 4th year here at my school who did the same thing and worked all throughout med school and my Big who is a 3rd year is also a PT and is doing it as well.
Anyway, the money is excellent as a per diem and that is definitely helpful. I only worked 3-4x/month at the most. I also am interested in ortho and pm&r so I thought I'd get in the hospital to learn my way around prior to 3rd year and to find out more about the ortho docs. I would say for me it has been helpful to be a PT and let it be known to those docs, they usually pimp you better on questions that are relavent and if you know your stuff you can show up the more senior students SOMETIMES (don't get cocky though).
I don't think this took away at all from extracurricular experiences, if anything it added. You see I did some research with a pm&r doc between first and second year who works closely with an ortho doc here and although it wasn't ortho, the orthopod saw how hard i worked all summer and being a PT definitely got me the position.
If you want to do it I'd say go for it. Be pro-active and get to know the orthopods in your program while you're working and let them know you're a first year and a PT (face time is huge, at least at my program - go to any morning conferences you can if you're allowed, with your background you'll at least be able to get something out of them and be interested). early interest is very important for ortho so if they see you're a hard working person interested from day one it can only help. Your friends will be jealous of the money you can make in one day and you'll be able to keep up your skills in the field you originally trained in (especially important to keep up for ortho since your mskel/neuro physical exam will run circles around any of your classmates until 4th year, if they're lucky enough to learn from a ortho/physiatrist/neuro).
I would definitely do it again if I went back to the beginning. Just be real good about time management and you should be able to fit everything in just fine. keep reading this site in the ortho forum and go to
www.orthogate.com and register for their forum and you'll get more good info throughout your journey.
best of luck!
-J