The BTC exchange rate rising should pay for the site just fine.
But the bitcoin project itself have no bitcoin from which they can pay for the cost. Sirius is paying for hosting on his own money.
Then it's up to Sirius to make any calls regarding financing, of course. I don't know how long Sirius has been involved, but probably a good while. Anyone who's been around from early on would likely have quite a few BTC stashed, so running bitcoin.org is probably a decent investment, assuming the project gains in popularity.