I don't know, I haven't ever really had any relationships that have lasted long enough for me to cheat, but it wouldn't nessecarily surprise me to find out that I'm capable of it. :/ I would like to think I wouldn't, but don't we all?
As far as cheating on tests and the like, I have been known to sneak sideways glances. But always on minor and unimportant things (like board games with friends). I won't on anything major.