In reply to gazhbo:
I got pain in my arm and shoulder when I started running too. For me, it was mostly about being too tense - when I relaxed it went away.
I've been running regularly for about 4 years now and I still get it sometimes, but usually only when I am worrying about something else which makes me tense - or at the end of a race when I am pulling all the stops out.
I also used to get intercostal cramps which also went away when I started relaxing more and were probably also related to being tense. I also have one sports bra that seems to cause me similar problems, although looking at your profile, I doubt that's your problem!
As for how to solve it, if I am racing I tend to just push through it because it always backs off pretty quick once I stop. Mental distraction techniques work pretty well for me - if I focus on the pain I just get more tense and it gets worse so I play stupid games in my head instead.
If I am doing a training run and just getting some miles in without the time being important, I will often stop and swing my arms around a bit to loosen them up. Making sure I am breathing smoothly and all the way out seems to help me and sometimes attempting to change my breathing pattern slightly helps (this may just be because it's distracting though).
I also spent some time focussing on my running form - I used to run with quite hunched shoulders, especially when I was going all out, and the thing that has helped me most has been focussing on relaxing my upper body and making sure that my shoulders are dropped and not pulled forwards when I run. I suspect that this is what goes wrong when I still get pain when pushing it for a race.