I've been using 4/8/16/32GB cards on my D90 with no problems. The 16 and 32 were the first ones i bought, when i didn´t know that is safer to have several 4/8GB cards in case one of them fails. All the cards i bought lately are 4 or 8 GB to use on the D90 and D700.
Nikonians!!! My best investment made after my camera!!!