@John: thanks for the suggestion, but get a bit too big, it needs to fit inside a wristband. Or are there very flat motors that would do the job?
What is going on for the user here?? Is it something like a heart rate monitor??
The idea is simple: it's a "Boost Converter" often used in power supplies. See: http://en.wikipedia.org/wiki/Boost_converter I think all you'd need is the inductor and a small transistor to act as the switch.
You said "up to an inch (25mm) in diameter". Here's a nice little 5V DC motor with a 12mm diameter body (10mm across flats):http://www.digikey.com/product-detail/en/M1N10FB11G/P14354-ND/2417078
Some people cannibalise old toys to find them