No. The only thing that this key can do is signing an alert message that propagates through the network.
Learn to read. It's about a line of text, an informational message, and only affects the reference Bitcoin-Qt implementation (most Bitcoin users are using different wallets for their routine needs)
The worst that the FBI can do if they waterboarded Gavin to get the key is speading FUD to Bitcoin-Qt like "All Your Coinz Are Belong To Us!"
It also can be used for sending the upgrading message. Remember the last bitcoin issue(Hard fork)? Did you read the source code before your upgrading?
I just want to say, if you trust Bitcoin, you need to trust the core developers. I did not say they are not trusted.