I don't think it's that recent, I feel like they've been pretty 'wealthy' for a while. Interestingly, NOK-EUR YTD hasn't been that great http://ur1.ca/fuyz4
I have an impression that they're are saving oil/profits for some possible "black day" in the future. And, the "profits" which are shared among all citizens come from high taxes. I might be [partially] wrong though.
Well, a large part of the oil revenues are put into an investment fund because the oil isn't going to last forever. Profits of that fund go into the public budget, though.
According to Wikipedia, there are discussions about how much of the oil revenues should be saved; opponents of using more of them directly say that it would just lead to inflation.