I tried changing the bulb in my Soubitez, but the screw holding the casing was a tiny cross-head one, and my swiss-army knife was too big to undo it. Doh, you have to change these things by the side of the road using a poxy little battery lamp, usually in the rain and wind, so lets put a microscopic screw in there for fun...

Keep a spare bulb in a film canister, or wrapped in foam inside your handlebars or seatpost.