Two or three years for a compact, and four-ish years for a dSLR, is not too old for a digital camera to be a good buy, in my opinion and experience.
Lee:
For one thing the price of the latest and greatest incarnation will most likely be less than the older camera was.
But it won't be less than what the older camera is now. In most cases, the price falls as a model gets older, to the point of becoming cheap after it's been discontinued, especially on the second-hand market.
Of course it won't have all the latest technologogy, but an ex high-end model may be more satisfying than a new bottom-of-the-range model if it has better ergonomics and ease-of-use. In some cases even the image quality will be better if it has a better lens or less noisy sensor thanks to fewer pixels.
The digital cameras I've bought in order are: Pentax Optio s5i, Pentax *ist DS, Pentax K10D, Panasonic LX2, Pentax K20D (last one not quite bought yet). All of these I could not have afforded when they were first introduced, and all were better for me than anything newer at the time. Two were second-hand, the rest were new at greatly knocked down prices compared to the initial prices. The total difference was well over £1000.
Early-adopting is a game for rich people, and I ain't rich, yet I have managed to aquire some very good stuff by being one or two steps behind. Same with bike components as well.
Actually I'm finally coming up to date with the K20D, but it won't be long before Pentax bring out a new model (probably costing at least £400 more at first), then I'll be happily one step behind again.