Marvel comics-inspired NBA team covers appeared on ESPN The Magazine's NBA preview edition in the first of the NBA-Marvel partnering efforts. I'm guessing those covers were a success because the NBA and Marvel have just announced that they are launching a new line of clothing pairing famous Marvel superheroes with famous NBA teams. Imagine the Hulk and the Celtics (think green) and Spider-Man and the Knicks (the webslinger prowls Gotham). Since the pairing is only famous superheroes and famous teams, I imagine that means we won't be seeing the Memphis Grizzlies paired with a superhero anytime soon.
Without seeing the clothing line, it's a bit premature to speculate on the success of the venture. However, Marvel characters are very popular now, fueled by the success of the recent run of Marvel movies: Spider-Man (and sequels), X-Men (and sequels), Wolverine and Iron Man I and II, with more on the way (Thor, Captain America and The Avengers). The saturation marketing movies seem to get coupled with the ever-present NBA ought to guarantee success. The clothing line just has to not be too ugly to succeed.
One other point about this partnership that we should not overlook is the deep and extensive partnership that the NBA has with Marvel's parent corporation, Disney. You may recall that Disney is also the owner of ESPN and ABC, two of the NBA's longstanding broadcast partners, so this is just another exercise of corporate synergy at work.