Ken Kifer's page on this is great. It's a ton of information, some of which he admits isn't very conclusive. But he basically just says: It's pretty much about as dangerous as everything else, but it won't give you clogged arteries or a bad mood.
http://www.kenkifer.com/bikepages/health/risks.htm

Unfortunately, Mr. Kifer was killed by a drunk driver 5 years ago while on his bike.