The NBA is its own market. Where else are tall lanky people going to work? The players cannot compete in baseball (check out MJ's attempt at baseball) they cannot play football. So not only is the NBA unique for the consumer, it is unique for labor. Oh, and I would really like the NBA to go into court and argue that the NCAA is its competition in basketball, because that will once and for all destroy the concept of NCAA basketball as amateurism.
You are right Salt the bulk of the anti-trust litigation will be defining the product market. And the decision will lie largely not on the facts but on the ideology of the judge and to some extent the jury.
I believe the product market should be restricted to basketball.
I don't think Jordan's inability to play baseball at above a low minor league level has anything to do with anything.
Danny Ainge was a pretty [dang] good baseball player so there's one anecdotal example that defeats yours.

Dave Winfield was recruited in three sports and almost certainly could have played professionally in any of them.
There are a ton of former basketball players playing tight end and receiver in the NFL.
Heck, Zdeno Chara is 6' 9" and playing _hockey_ for gosh sakes.
And the fact is, there are other professional basketball leagues around the world (Spain, Turkey, Russia, China, etc.). Lots of american ball players are already playing in those leagues.
So making a anti-trust argument around restraint of trade seems like it would be pretty weak.