I'll echo what RPG Pundit said.

The essential problem with basic sets is the bind that TSR found itself in:  If the basic game is good and worth playing, then consumers will demand more of it and won't upgrade to the more complex system.  If the basic game is bad, if it is crippleware (great term!), then consumers won't upgrade to the more complex system.

A better idea is to produce something like HERO Games Sidekick, which presumes you have the full system and want a guide to the system, and doesn't advertise itself as a "basic game" but rather a "quick start rules" for the main system.

Though D&D 4E shouldn't need this at all, because if 4E is as complex and intimidating as the HERO 5th Ed., then someone fucked up royally.