Encouraging early registration a good idea. Unfortunately, financial discount ain't gonna cut it (Sun also extended their "early bird" discount right up to the day before JavaOne, "in light of the current financial circumstances").

Perhaps some form of limited merchandising, possibly also available to buy? A stylish jacket, carry case or whatever. Something which'd be too pricey to give out to hundreds of people, but the first 100 registrants get a $NICE_THING.