Spain adopted a mandatory helmet law for cycling outside of cities in 2004. Helmets are not compulsory in towns and may be removed while climbing steep hills.France has a lively discussion on helmets going on. The best summary is probably this page on the Mieux se Déplacer à Bicyclette site. They analyze deaths in Paris and in France as a whole and conclude that helmet usage is a personal question but can save lives.
I began reading Randy Cohen's The Ethicist column in The New York Times Magazine when a friend pointed me to his discussion of (motorcycle) helmet laws. Although he tends to the snappy and glib, he was spot-on in saying that the financial and social cost borne by people other than the helmet-wearer offsets the personal liberties infringed-upon by requiring helmets.
I still shudder when riding with someone who isn't wearing a helmet. I think I value my noggin and what's inside, and will take all reasonable precautions to protect it. On the other hand, I rarely wear gloves when cycling, and I learned a hard lesson a few years back when I tumbled down a briar-covered hillside in Joaquin Miller Park.
Some of this may have its root in my cracking open a Kiwi helmet when I was 11 or 12, and walking my 10-speed bicycle across a busy intersection when an elderly Unitarian barrelled around the corner of Beechwood and Wilkins and sent me flying. Of course, not everyone who wears a helmet is doing so sensibly: I cringe when I see people flying down Haight St. with their helmets perched ever-so-saintly on their heads, but fail to stop at the intersection (controlled by a STOP sign), or wearing headphones, or both. Fer crying out loud: I've seen people eat a chrome meal at that intersection, and it ain't pretty.
Somewhere I have a picture of me in Barceloneta, 2001, on a bicycle but without a helmet.