"There's been a bit of tension for some time," says a source close to 30-year-old John. "They initially opted for a break, hoping a trial split might make them stronger. Sadly it doesn't seem to have worked." Rumours also included Jennifer wanting more from the relationship than John was willing to give her. (I don't think asking to be called by one's first name rather than "baby" or "sweetie" is too much to ask for, since you know that bastard can't keep them all straight.)
Time will tell if this is actually true or not, but going by both Mayer and Aniston's respective track records, I think we can all safely say that we were surprised they made it this long.