In reply to Chris the Tall:
I have a road helmet for road and a mountain bike one with a peak for off-road, which I usually also use on the hybrid, as it's a bit more "casual" style.
Among other things, mountain bike ones tend to have smaller vents to stop penetration from rocks and twigs - a road helmet can generally be expected to hit a flatter, more uniform surface.
My old road helmet is a Bell Sweep R, which was developed with one of the pro road teams and got very good reviews. They also released a Sweep XC, the difference being it had a peak and cost another £10 - I saw one review of it in a mountain bike mag that panned it, concluding "it's a road helmet with a peak"!