My DS is almost 6 months and I have noticed a couple of small bruises on him. He hasn't had any major falls but we are doing more rough and tumble with him than we did when he was small (during which he laughs out loud), but it is still gentle by anyone's standards (of course he has fallen a couple of times whilst trying to sit up but nothing bad enough to even provoke a cry).
I am a natural worrier and I started worrying about leukemia (he had a small bit of blood in his snot the other day too). Obviously, the answer is 'make a doc's appt' just to be safe, which I am going to do. But meantime, are bruises on a 6 month old common? One of my mates was surprised and said that babies don't tend to bruise unless they really hurt themselves - or are ill.