Quck update: Coinbase no longer seems tied to Bitstamp, at least not this very moment. Both the buy and sell prices are above Bitstamp's. They're either making a hard buy themselves, or are tracking to another system. Any ideas?
Will try to back my findings up with new charts, soon.
I just started to notice this the other day as well. I figured since they were a BTC/USD exchange they would just establish a market rate themselves. The only thing I don't like about coinbase is you can't put in reserve buy/sell orders at a given market price. Meh, other than that I like coinbase a lot, very stable and reliable.