From what I've read, Nikon service centers in the US won't even touch import/gray market lenses, although you'd still be able to get them repaired at non Nikon service centers. Basically you have no warranty coverage. The 1yr warranty that applies to imported stuff only applies in the country it was imported from.
For goods marked 'Imp' instead of USA, here is what B&H say:
"For IMP items only, B&H provides a warranty identical to the provisions and limitations of the manufacturer's warranty for such items, with the exception of the time period, which is equal to the term of the manufacturer's warranty or one (1) year, whichever is less."
The complete explanation of the differences between USA and directly imported items at B&H is here: link.
It's well worth reading if you are considering buying directly imported equipment from B&H.