I think B&H sells lenses with 1 year International warranty + 4 year extended Nikon USA warranty...means that for 1 year you get warranty anywhere u go, then the next 4 years u have to go back to USA if u wanna service...
But anyway any lens from Nikon (not sure about the other brands) come with one year international warranty, standard one. So you're safe if u buy from B&H. Not very sure how Adorama/Ritz/other stores handle their warranty, but if they get their lenses directly from Nikon, they will come with 1 year international warranty.
Imported/grey sets will have no international warranty i think. In fact Nikon warranty will be void if it's a grey set. However some stores will provide their own warranty, but I think not worth to send to US for repair.
Refurbished stuff will come with 90 day limited warranty, but I'm not very sure what that covers, and if it's international.