I just received mine today and this is a "white box" lens. It doesn't come in standard retail packaging (just a white box) but it does come with a USA warranty card inside and I was able to register the lens's serial number on Canon's warranty site. Probably this was originally a kit lens for a 5D Mark III. Amazon ought to note this in the product listing.
Mine also came in a white box but the box was labeled grey market and no warranty card was enclosed. It was purchased direct from Amazon. I called Canon and was told they will not provide any warranty for a grey market lens.