Yeah I noticed this. Is there any exchange with higher prices than Mt.Gox?
I think this is because Coinbase will "lend" you money during the transfer gap.
For example I can go to Coinbase and lock in a price immediately even though it takes several days for the USD to go through the system. Alternatively I can wire money to bitstamp or mtgox, wait a day or two and then accept whatever the price has moved to.
I made a Coinbase purchase a couple days ago and originially was annoyed by the ~2% premium over bitstamp, but in the end I received a better price than if I had wired money to bitstamp and puchased after the delay.
It's interesting to note that when the price was stable Coinbase better reflected bitstamp prices. IMHO I think Coinbase will tend be priced higher than exchanges during rising periods due to the ability to immediately execute prior to funds clearing.
