Could you explain what you mean by habitable? Is that in that you can walk around with relative safety in certain places, or that you can safely live their again?
I'd guess the latter. The way half-life works, the stuff that's dangerous to you as you walk around quickly decays and stops being dangerous.
The problem is with long half-life (and thus low-radioactivity) elements that sit on the trees, get mixed with soil, and thus eventually end up in ground water and local food chain. You might be able to walk around a disaster site safely in relatively short time after the event, but I wouldn't try growing anything there.
