An interesting question, the answer to which I have not a clue...
As Mucks pointed out, you can still buy a PCI card.
As for Chris assuming future AGP cards will be dearer and slower, dunno, the current range of PCI cards are about the same price, and they're only slower because of the bus they have to use.
So I'd predict: They will be available for some considerable time yet; the only reason they will be slower is because sli is faster than AGP; and they'll probably cost about the same as it costs the manufacturer about the same to make them.
I must admit I am surprised there are still so many PCI cards about, I suppose there must still be a market for them.
Much the same way, perhaps, that there will still be a market for AGP cards in 2009.
Whether new design cards will still be available in AGP format, we'll have to wait and see, but my guess is that we'll see at least a couple of cards in AGP format to surpass the current top two contenders before manufacturers go over to sli completely for new models.