I'm reading this with great interest, I'm not Jewish but people regularly assume that I am. I even had a boss give me a Rosh Hashanah card--I emailed her thanking her for her kind thoughts but explaining that it isn't actually my holiday!
So, I'll second what eternalsunshine said, there *is* a meaning to "looking Jewish" even if it isn't realistically tied to *being* Jewish. In my case, I'm part Lebanese. I look Semitic in the larger sense, but in an area where there are a lot of Jewish people and not many Lebanese, Semitic means Jewish to most people.
Funny part to me is that I've had a number of blond, blue eyed Jewish friends, including a college roommate with the most beautiful silky, naturally straight hair--again, looking Jewish isn't necessairly related to being Jewish, but to many people, there is a set of physical characteristics they tie to it.