You know, when I first started playing this game, I really overlooked Liara, probably being more focused on Garrus and Wrex. So I never really went ahead with any of the romances in the first one, though I flirted with the idea of Miranda's in the second. Anyways, I ended up going back to play both games straight through. The rest is history.
I gave her that chance she never had. Maybe because we're cut of the same cloth now -- we're both archaeologists (or I will be once, once I'm freed from the clutches of college).
But I've really come to appreciate Liara on subsequent playthroughs, and the main Shepard I carry into ME3 will have romanced her. She just seems "right" for him. Not that it hurts that she's worked her way up to be my favorite, right there with Garrus.
To answer the above question, though I'm sure you're all more familiar with Liara's dialogue trees than I am... I think one of the most amusing exchanges between Shepard and Liara (and possibly in the entire game) is when the two of them are discussing why Liara has been so reserved; Shepard -- as the bottom right choice -- has the option of saying, "I'm ready now ... if you are." As I recall, Liara just kind of retorts back, "Eager is not the same as ready, Shepard." Not the route I normally go, but it cracks me up every time. Absolutely priceless delivery.