Tittering aside, this is actually a pretty interesting question, one that I've discussed with women runners. After all, if we're born to run, why do many women need jog bras in order to actually run? As the study notes, it can hurt.
I've surmised that barefoot-style running, which involves less vertical movement that heel-striking, and hence less bouncing, would be less painful for woman runners.
Although there's also the possibility that large breasts evolved as a secondary sexual signal after tribes of people settled down a bit, and women didn't run as consistently as the men did. After all, in most hunter/gatherer societies, the men go out and hunt, and women raise the children and do the gathering.
If the ladies have any thoughts on whether breast bouncing while running improves because you're running barefoot or in barefoot-style shoes, do let me know in the comments below, as it's a pretty interesting topic for women runners.
At least one woman has found this to be the case... All that said, the study Pete links to falls under the "duh" category of research:
"CONCLUSIONS: Bare-breasted kinematics significantly increased with cup size during running. Differences in breast displacement, velocity, and acceleration between cup sizes could be predicted using estimates of breast mass based on conventional brassiere sizing...."No kidding. I think any man or woman could have explained this to the researchers. They didn't really increase the scope of human knowledge with this study...