Just because a camera tops out at 30MB/sec does not mean it'll fully utilise 30MB/sec cards.
I used 4GB Extreme III 30MB/sec cards in my D200, which gets the D200 running at its max speed (which is WELL below 30MB/sec). When I put those cards into the D300s, they topped out around 22MB/sec.
Going to the 60MB/sec Extreme cards got my camera writing out at full speed.
There are comparison charts out there for transfer speeds with different bodies using different cards. I don't remember the URL off-hand, but a quick Google search should bring them up.