With the 1.5 update, I've been looking at some of the new minecart features. I'm looking to replace my "traditional" boosters with powered rail, which should be easy enough, but I've also incorporated auto return for empty carts. If detector rail activates when empty carts roll over it, then it obviously won't work, and I'll have to continue using traditional boosters, at least for the stations. The mincraft wiki isn't clear whether detector rail will activate or not.
Under most circumstances, detector rails will not care whether a cart is empty or not, and will trigger. However, if you put a detector rail on a slope (so the detector rail piece is angled) and the following section of track slopes the opposite way, the detector will not trigger for empty carts!
Bear in mind that detector rails on slopes are said to be a bit unreliable, and that this behavior may be considered a glitch and thus "fixed" in a future update!