This is a topic that has garnered much attention in the media especially following a wsj article last year on the concerns that training for distance running was bad for your heart (due to the cumulative effects of a long term inflammatory state) - I really like this blog written by an electrophysiologist and bike racer he has a few well balanced things to sat.

Also, he notes Steve Tilford's blog in his page, which can be found here

To me this is fascinating science and underlines a few things I've thought for a while - rest and recovery is important, balance is good, but on the whole I'm left to conclude that for the majority of folks on this board it should make no difference, this is certainly not the same thing as saying that climbing Ham at Threshold is likely to cause an MI. It's more about long term stresses and lifestyle.

My $0.02