I've been refraining from weighing in on this topic for most of the past week, because I never play goalkeeper on any serious level to really say what would work from what wouldn't. However, I played enough basketball in college that I could have minored in the sport... the amount of hours I spent on the court. I never played scholastically, but I played enough ball competitively on the intramural and rec league level to add to the discussion. I would be surprised if anything could be learned by a prospective goalkeeper, other than maybe some footwork.... maybe.
I like Elan's suggestion of tennis, b/c even though I never thought of it, it makes perfect sense with all of the lateral movement involved, including quick change of direction, and the demands of having to quickly go from crouching to full-stretch etc. Not to sound contrary, but boxing out in basketball is a lot different from positioning in the box on crosses. Boxing out requires you to get a firm (immovable) base and utilize your leverage against your opponent. Contrary to what some may think, it's not about outjumping the other guy... there'll always be some guy who could jump out the gym as they say, who will beat you to the ball if all you try to do is out-jump the next man. Charles Oakley could barely get three inches off the floor, yet was an excellent rebounder because he understood positioning and leverage. Meanwhile a leaper like Shawn Kemp (or even better, Kenny 'Sky' Walker) was a weak rebounder b/c he didn't understand how to battle on the boards.
Any racquet sport would be a sure bet in my book for all the above reasons related to tennis. I would even go a step further and say 'handball' would be even better. Not that nonsense you see for the Olympics (although for all I know it may help), but rather the "hand-ball-wall" game we play primarily in NYC. The agility you get from that game is crazy. That to me develops the hand-eye coordination that many refer to... baskeball won't. The basketball is too big and moves too slow to adequately develop the type of reflexes (I assume are) necessary for goalkeeping. Further, the baskeball is too heavy to sufficiently prepare you for catching a football. A regulation sized basketball weighs about 1.5 pounds... whereas FIFA regulations stipulate that the football weigh around 0.875 pounds (14-16 ounces). Almost half the weight of a basketball. That's like arguing that playing with a medicine ball will make you a better basketball player. Might make you stronger, but not better.