The Canon 550D is a superb camera from what I hear. I haven't owned one but I want to pick one up for a secondary camera.
I've owned the 60D, it too is a superb camera, but I really can't help but think that it is aimed at those wanting to make videos from their DSLR.
My opinion is to stick with your 550D because I don't think you'll notice a big change from purchasing a 60D or 7D.
I'm not saying that a 60D or a 7D is not better than a 550D because of course normally the newer, the better. But I just don't think you will notice a lot of improvements from your 550D.
My advice is to wait another year before upgrading your body otherwise you might end up losing out.
Thanks!