I can't believe this even has to be said in the year 2007 (almost 2008!), but I've seen too many counterexamples in the last few months:
PLEASE, do yourself a favor and wear a helmet while cycling. The cost/benefit analysis for bicycle helmet usage is beyond a "no-brainer."
For visual impact I will include this photo from a German bicycle safety campaign with the slogan "Be careful when transporting fragile goods"