Scam or real?
Skepticism is good, IMO. There are very good and ethical contractors out there, and there are ones that will take you to the bank. The construction business is not unique in this respect.
That said, there's no way for anyone here to justifiably second guess your contractor based on the very limited information you provided. You've described the problem fairly well, but we need more details about his recommendation and how he came to that recommendation. If you're willing to provide those, you might be able to get some opinions here. Those opinions should not be used in lieu of a second opinion from another contractor that goes on site and examines your situation; it should be seen as supplementary.
Now, generally speaking, a french drain along the interior perimeter of a foundation wall is not uncommon. Such a drain belongs on the exterior side of the foundation, but circumstance sometimes prevents a replacement drain (or new one) from being installed there--typically, a neighboring structure that prevents adequate access. That said, you never said where that contractor said you need the french drain. This gets us back to the problem of limited information. Provide details, and you might get some helpful feedback. In the absence of it, all you're going to get are haphazard guesses.
Last edited by cortell; 05-11-2012 at 10:26 AM.