Opinion seems very polarised on this issue, with some aggrieved at the launch of a new model so soon whilst others are of the opinion that what you bought is still as good as it was before the new model was announced and you have no right to be upset about it.
Personally, I think the issue this time is with the secrecy surrounding the iPad4 and the fact that there was very little info or even rumour about its release until very recently. Whilst all technology is constantly upgraded/replaced, at least with most technology, such as processors, graphics cards and so on, we know what's coming well in advance and are thus able to make an informed decision on whether to buy or wait. Even with the iPad3, we all knew it was coming a good few months before it actually arrived, so you can decide to wait if you want to.
With this model though, even a month ago there was virtually no reason to believe a new model was coming, so people had no reason to wait and bought the then current iPad3. I think the annoyance comes from the fact that, had they known the new model was coming, they'd have been able to make an informed decision on whether to wait or not and it's this that feels like a slap in the face.
Imagine if car manufacturers gave no notice of new models and kept them completely secret then, a couple of months after you pay out £20k for your shiny new car, it's replaced with a new, improved model. How annoyed would you be at that?!