I needed to convert some units a long while ago and found this great little app to do it. Here. Its written by a guy called Josh Madison so kudos to him for writing it and releasing it for free :D
I used his app as an idea to write my own version for the Pocket PC, back in the days when I had an iPaq. Mine is here, also free to download and has the source as well if your interested in that sort of thing.
Thought about updating re writing mine to work on the iPhone, but turns out you need a mac to run the SDK on :/ Any one want to give me a mac so i can do this? No i thought not.