I would say, both of them. Higher demand then supplies and speculation. But the problem with speculation is, its not only specullation on tight supplies, its speculation on dollar weakness.
from my another post here:
http://www.telegraph.co.uk/money/mai.../ccview109.xml
"Crude is now moving almost reflexively as a sort of "anti-dollar", a currency on steroids with eight times leverage. No matter that the global economy is slowing hard. Bad is good for oil in the topsy-turvy world of commodity funds."
Now everyone and his gradmamma are speculating in oil on futher dollar debasment.
This is what you got, when you have FED and Treasury trying by low interest rate policy, to bailout wallstreet banks, without any respect to average Joe, while those yet to bankrupted banks are trying now to get out of their own mess by playing a commodity boom too..
Of course there supply problems too, but the speculation factor is big too as we can see on $16 surge in the price of oil on small dollar devaluation in a two days two weeks ago
The Central Eurobank head told it few weeks ago: "to cut interest rates at this point would be the same as taxing the people to bail out the banks. Cost of extra inflation would be the ultimate sign of moral hazard to save banks."
In addition to supply problems, now you guys in the US got a pay for monetary policy of your institutions and we too as oil simply moves also as currency hedge with huge leverage.