Are coinbase.com purchases effectively bitcoin futures? I just purchased 0.1 BTC from coinbase with a bank transfer. Coinbase informed me that the transaction would take 4 business days to complete.

So I don't have any bitcoins yet, but Coinbase promises to deliver 0.1 BTC to me in 4 days.

Coinbase probably didn't purchase the bitcoin now because my payment hasn't cleared, but if they make the purchase in 4 business days from now and the price has gone up from $330 to $500, how will Coinbase cover the difference when the coins need to be purchased?

Or does Coinbase just regularly purchase bitcoin and then just transfer some into my account when my funds clear? Or do they not actually give me the bitcoin until I decide to sell it?
One they credit your account you can transfer your bitcoin out to another wallet, you do not have to sell it through them, but you could at a later time if you wanted to.

Unfortunately Coinbase is also known to just cancel an order if the price sharply rises between you placing it and them delivering the bitcoin, your US dollars are then returned to your bank.