Good question - never seen one. I really don't pay much attention to be honest. The pro-lenses I own have the the gasket at the mount. It is a good bet that all the $1k+ lenses do.
I would not call it "sealed" maybe weather resistant. I wouldn't expect the same sealed like the Olympus E-5 if you follow other brands. Basically it just means that if it get's rained on for a short time or if you are in blowing dust, it will not get into the camera. (as long as you keep the lens on.) ;)
Personally I wouldn't worry about lens choices based on a gasket but more on what your needs as a photographer are.
I get that it isn't really weather sealed, such as bullet proof vests aren't 100% bullet proof.
Also I am not saying I only want to buy weather "sealed" lenses. Heck right now I don't even own any Nikon glass (NAS is kicking in) so it's not a real big concern for but I figure I want at least one lens that is weather sealed since it's a feature the camera has, so why not utilize it.
As for my needs, I really have none. I do this as a pure hobby, not for money (sometimes I wish I did, it would make purchases less painful to the wallet!) but I do seem to like the telephoto end which is why I purchased the Tokina 80-400.
Sadly it's just too darn slow for me and I've been trying to sell it + other stuff to get the D7000. Luckily sold off my D80 for a good price, so the D7000 just needs to be released but it leaves me kind of torn.
Once I sell my D90 I could either buy the Nikon 105mm VR micro and keep the 80-400 or sell the 80-400 dirt cheap and get the Nikon 80-200 AF-S + TC or just the Nikon 70-200VRI.
I'm hoping I can help narrow my choice by seeing which one is weather sealed or not.
Right now I know that I'll miss the long end and with the Tokina 16-50 my wide end is covered.