[jmatta]

After replacing the battery, you'll likely need to inset the key in the ignition and turn to on (not engaging the starter); this re-syncs the electronics (don't ask me why, but I had to do this when I replaced my battery; it is explained in the owner's manual). Upon removal from the ignition, you must press the lock button within six seconds to reset. It should work just fine from there.

Others have let the battery die or come close and it creates all sorts of problems (just read some of the other posts). My friend's key on his '02 was dying and the dealer replaced it just in time before the immobilizer took over, rendering the car undrivable.