As others have said, far safer to buy four 4GB cards than one monster 16GB . I still only use 2GB cards (about 200 pics on my D90), might upgrade to 4GB at some point but wouldn't go any bigger than that.
Isn't this like the whole hard disk RAID argument? Surely four cards means a four times higher risk of failure, but losing only a quarter as much? I'm sure someone even geekier could tell me!
Lose the 16GB and you've lost everything, or if the 16GB breaks you've lost everything. At least with a four 4GBs you are pretty much guaranteed to come home with 3/4s of your photos.
If I went to New York, I would rather come home with something rather than nothing, having more cards gives you that security (unless you lose all your gear!)