Waiting about two months after the initial release basically lets the major bugs be worked out. However, there is an adage that you need to remember whenever you get ready to buy tech. You should buy the tech when you need it, because new tech is always around the corner. If you keep waiting for new tech, you'll end up waiting forever.MacBook Air. It was just released, and the technology is adequate. The faster flash-based storage makes up for the slower processor. If you're deciding between a MacBook Pro and a MacBook Air, then if you can wait for your MacBook Pro purchase you should, as Apple will refresh the MacBook Pros next year. It's uncertain when exactly, but it could be Q1 or Q3 2011.
If you can't wait, then you should just get the latest MacBook Pro, boosted with as much as you can afford. The new MacBook Pros will have cheaper solid state drive technology, kind of like the new MacBook Airs, which will be very appealing since the laptops will be quicker, more responsive, and have the 'instant on' that Apple keeps talking about. Also, the whole MacBook Pro should receive an update next year.
As for cameras, the product life cycles are longer than computers. The tech is updated every few years and within six months of a new camera's release is usually the optimal time to buy one. If you're a smart consumer, then you can pick up older tech for a lot cheaper once newer models are released. They can be bought off Amazon or eBay at significant discounts.