I think Janissy is pretty much right, but I'd make a couple changes to differentiate passionate from friendly eye contact.
Typical friendly eye contact is only maintained a short time before looking away, maybe about a second? (Haven't really timed it...maybe someone else knows?). Passionate eye contact is held for longer.
The more important difference is that friendly eye contact is aimed at your general eye area, and attention isn't very fixed or direct. In passionate eye contact, attention is fixed directly and intensely into your eyes. The energy level of it is totally different.
Smiles can accompany either one. With passionate eye contact, whatever the expression, the energy level should be turned way up. Everything is amplified.
I believe this is true for both genders, although women tend to look away more frequently and do the whole batting the eyelashes/looking over the shoulder thing more (yuck). What do you think?
This system isn't foolproof, though. Every so often, you'll encounter someone who makes the sort of eye contact that typically indicates romantic interest, but they're actually not interested at all. AFAIK, though, I've only ever met one of these, so I think the track record is pretty good.
BTW, I really like the original question.