Well, this sort of question requires details about who this other person would be. Because if you feel like you kinda just ended up in the relationship by default and suddenly along comes someone who you click with unbelievably well, loves everything you love, finds you ridiculously interesting, looks ultra cute, but simultaneously is ridiculously interesting and intelligent themselves, well...I guess I wouldn't actually cheat even then, but only because I'd probably break up with my SO before making my true love the official new SO.
Of course, I'm probably a lot freer to say this than most of you are since most of you probably HAVE SO's and wouldn't care for them to see your post saying anything sketchy.
Aside from this rare circumstance of finding yourself in a relationship in which you aren't really in love and finding your true love one day, no I would never ever ever cheat, no matter how hot the girl is. Not even Anna Kournikova, and not even Jessica Alba.
Wait...did I just say that? Hmm does it count as cheating if you call your girlfriend up and tell her that Jessica Alba wants to sleep with you, and then BEG her to let you? LOL
Cuz I mean...if your girlfriend is cool that might fly lol.