This sort of behaviour disappoints me and does nothing to help the perceived smugness of mac users. I was detered from the Mac by what I considered their superiority complex and it was only when I eventually tried the product that I changed my mind.
It always strikes me as odd that someone would base something as fundamental as the computer they use on an image of other people using that tool. It reeks of a grade school mentality.
IMHO, Apple should be "smug" (if that's what you need to call it),
and they should back that smugness up with a "this is how you do <X>" second wave of advertising when Leopard comes out (or even now using Tiger as the comparison). As others have pointed out, the only time you shy away from competitive comparisons are:
1. When you have the market sown up, and so mention of the competition is publicity for the competition
2. When you have serious issues in your product which pale in comparison to the minor issues you are pointing at in the competition
Apple is definitely not in the first area, and I don't see how the second applies to them. A direct attack on Vista flaws is certainly justified and, quite frankly, expected from a marketing perspective. As for timing, a direct attack is generally most effective:
1. When the competition is touting an advance or new version
2. Before bringing your own new version out which leapfrogs the competition in the critical areas.
Again, perfect timing here for Apple to go on the offense. Counters MS in a news cycle that they would otherwise own, and has the potential to grow into a major positive campaign on the release of Leopard.