My mistake for the accidental slip with 10.8.3, been working with a bunch of Lion machines in the shop lately and they were all running 10.7.3. That being said, I don't even think 10.8.3 is available on the Developer portal, yet. 😉 No more mention of that, though, otherwise the forum police will bear on this thread with the wrath of the strength of 10,000 retirees.
The issue exists in 10.8.1 as well from my experience. Our fleet of 13" Retina MacBook Pros exhibit the same issue that the OP is reporting (15-30 second shut down sequences). None of them run 10.8.2 because the update rendered the machines unusable, it's also why (I suspect) that Apple pulled the 10.8.2 for those machines. Personally I've noticed it on all the machines since the upgrade to Mt. Lion, it's just not that obvious on the MacBook Air due to the SSD (I suspect).
Personally I'm curious to know what is that's happening in the background, the OS (at that point) has been secured and shut down. The only thing I've been able to correlate is that the more inactive RAM I had allocated at the time of shut down the more time it takes to shut down. I shut down all of my programs before shutting down the machine so my wired RAM is usually sub 1 GB (on 8 GB of RAM). Perhaps Mt. Lion is doing something with that RAM, a secure wipe or some other system level task.
It's definitely a "feature" of Mt. Lion, though. Of the multiple machines I've tested that can run Lion and Mt. Lion the issue only occurs in Mt. Lion (dual boot).