Thanks people.
I don't intend upgrading soon. I don't have the funds. I just didn't understand how Canon's model numbers work.
For example, is the 550D better than the 50D or vice versa?
In features, yes the 550D is better than the 50D, but the 50D is built to a higher specification (which does have the unfortunate side effect of making it bigger and heavier).
It does become a little confused at the 550D level because a number of bodies use the same (or very similar) sensor. The 550D, 600D, 60D, 7D and now the new 650D.
From a build quality view the 7D is top of the pile, but IQ wise all are similar, from a technology point of view the new 650D would come out top.
It really is a matter of go and try the cameras somewhere, if you are used to the 1000D and like the size and weight then the 650D is currently the best upgrade you could do, if you want a bigger, heavier camera with a slightly better build, then there is the 60D, or another step up to the 7D.
Of course, you could also upgrade a further step up to full frame, and then the 5Dmk2 and 5Dmk3 could be considered, and another step up above them is the 1Dx (of course by this point you are up to a very large body weighing almost 1.5 kg and costing over £5000).