TouchRPN is a fully unit-aware scientific and engineering RPN calculator for iPhone. It is designed with a broad audience in mind, with tools that will appeal to a wide range of audiences. If you are unfamiliar with the RPN (or "reverse polish notation") way of doing calculations, I encourage you to become familiar with it! If you find it is not your style, at this point, TouchRPN has zero support for an algebraic ("normal") interface, though this may come in the future. To this end, you may want to try out a physical or software RPN calculators that are available, before you decide to buy TouchRPN.
Any units that can be reduced to the 7 SI units can be supported, and if I left it out, it's because I couldn't think of it when I wrote up the list of units. Let me know there's an important unit I left out, and I'll add it in on the next go-round.