Exactly! Release just before Vista and the excitement lasts a few days, until Vista comes out. Release just after Vista and there will be hardly any excitement because all the focus will be on the OS 90% of the world uses. Release at least a month (or maybe two) after Vista and OS X could be the answer for the people that ended up being disappointed with Vista.
The problem with this logic is that it simply will not work in the real world. Very few people, no matter how disappointed they are with Vista, are willing to go out a few months later and drop a grand or two on a new computer (in this case a Mac). Most people buy a new computer once every five years, if even.
There will be a flurry of new purchases whenever Vista comes out, and every one of those purchases will be a lost "switcher" for the next three to five years minimum. Unless of course Apple get 10.5 out the door and have a good "new" alternative to Vista, with features like Time Machine that everyone can use and Vista lacks.
Apple need to get Leopard out before, or near, Vista and push it hard as a better alternative to Vista. People are ready to change from XP to a new OS and Apple can capitalise on this or they can miss the boat by a few months and lose a lot of potential switchers because of Vista.