I'm recently returned from Jimani, a border town in the DR, where I worked at a private clinic-turned-hospital run by a Baptist group treating refugees from PAP. Most came by bus or were (egads) medevaced out to our pitifully tiny and resource-limited clinc/hospital. A few walked the 30 miles from PAP.
There was a patchwork of aid groups there staffing the place when I left, though it looked as though some of the larger groups were starting to take over (a good thing). Most of the volunteers there arrived very shortly after the quake and were experienced. A few started showing up later as freelancers and a few were from inexperienced aid groups. For the inexperienced, I'd say it was 50/50 whether they'd be useful or in the way/harmful. The ones who WERE useful (including most of my group) had a number of things in common -- several years of healthcare experience (not just RNs; we had an LPN and some paramedics with us, too; they were awesome), international experience, experience in resource-limited settings (hey, look! red rubber tubing and a milk bottle! I've got a chest tube!), and experience with "bad outcomes."
For inexperienced volunteers who want to go now: The latter is very important. You may "know," deep down in your heart of hearts, that you can handle watching someone die, watching a small child make sounds you've never heard a human being make while her mother weeps next to her, and that you can care for someone with severe facial burns and a field amputation. But honestly, if you've never seen anything like these things before, it's likely to have an effect on you and you don't know what the effect will be. You could be hurt mentally. You could develop an infection or other illness physically. You could find yourself spiritually compromised. All these things can still happen if you've been there, done that, of course; but then (hopefully) you know what you need in order to cope. Watching another volunteer decompensate and knowing that you're going to have to clean up THAT mess, too, is extremely frustrating -- especially when the other volunteer is a freelance newbie you've never met before.
That said, I walked into Jimani with no disaster experience, and no one in my team had any, either. We'd all seen death, grief, extraordinary wounds (I'm glad I saw my first exposed hamstring on an A&Ox3 individual in the comfy confines of the burn unit and not in the field, for sure), and international medicine before, and everyone slipped into their roles quickly and easily -- they'll need more people like that (and better) for a long time coming. Groups that would love to have experienced trauma people include partners in health (secular), LCMS World Relief (my group; they do expect you to be Christian or at least very comfortable with Christians, but there's no proselytizing), Operation Rainbow and Operation Smile. Good luck, all, and thank you for not forgetting Haiti!