Sure. Coinbase, as far as I know, sources its coins from the exchanges (Mt Gox, Bitstamp, btc-e, etc). I could be wrong about this.

Coinbase acts as a nice middleman where you can securely link your bank account, and for this convenience Coinbase charges you a higher fee than the exchanges. They procure your coins, handle the financial leg work, and suddenly you have access to bitcoins.

I wouldn't worry about it. I've used both quite a bit. It was a huge pain in the ass to gain access to Mt Gox. Coinbase is just better imo. The support team always comes through for any problems.