Thanks guys. I've somehow never thought of tyre differences. Come to think of it, my 700x25 road bike tyres are skinner, lighter and have less treads than the 700x28 hybrid ones. The tyres on my hybrid must be thicker too - they are labelled as "flat resistant."

That may also explain why I've never had a flat on my hybrid since I bought it last summer, but I just got one on my road bike in the first three weeks of ownership.