Bob Ames kindly wrote in response to discussion: I have a new MacBook Air, and a couple of spare Magsafe power adaptors from previous Apple laptops. As it seemed daft not to use them, I asked Apple support to clarify whether this would be safe, given the confusion experienced by others. They referred me to a very informative article, which gives specifications for all Magsafe adaptors for different Apple products.
It is quite safe to use an adaptor that can deliver the correct or greater wattage for your laptop: for example, if you have a MacBook that normally requires a 60 W adaptor, then you can use a 60 or 85 W adaptor, which will deliver sufficient power. However you could not use a 45 W adaptor, as it would not deliver sufficient power for that model.
Comments As it is stated clearly in this Apple article, there can be no doubt that using a different adaptor of suitable wattage will not invalidate any warranty or AppleCare cover.
Updated from the original, which was first published in MacUser volume 27 issue 7, 2011.