i never have an expectation of the guy paying, and i always have cash on hand to pay my share of whatever i'm doing, or i wouldn't go. i think that's a good idea anytime you're going on a date.
but in my experience, the guys wants to pay for the date. when that is the case, i typically make an effort to treat them in some fashion that is in recognition of that fact. so if he pays for dinner, i'll offer to treat for the movie tickets, etc.
more often, the guys rebuff that, however, some are more open to that--especially if it's after the first or second date.
which by the way, is kind of a delicate walk. it's kind of annoying to me how some guys (such as those in this thread) will talk about how they expect a woman to pay her fair share, yet, others feel quite the opposite--and i get the sense both sides would be offended if there was an expectation differently than what they prefer.
i think i've nearly offended guys in the past when i've tried/offered to pay for part of the date, and felt a little uneasy when i tried to pay for things in later dates. in my experience, most guys want to pay for dates but the majority are fine with me contributing in smaller ways, such as a trip to the concession bar for a couple beers to enjoy during a movie, etc.
after i've been out with a guy a handful of times, i'll usually treat him to a nice home cooked dinner-- as a way to show my appreciation, as well as enjoy a different kind of date.
i think it's important not to make assumptions, and if you're in a relationship with a guy who does tend to pay for things, that you are sensitive for opportunities to contribute.
and i agree, once you're in a relationship, there is a lot of opportunity for more equality and recognition of sharing costs or division of the shared expenses.