Oh what a great thread!
to distract me from my studies...xD
This is a question that keeps me up at night XD
Normally, I'm someone that believes falling in love takes time, it's a feeling that grows slowly as people connect with each other. Ulquiorra and Orihime definitely connected in ways that neither of them had ever experienced before, but it's hard to actually put a name to the feelings they developed. (In Ulquiorra's case, it's fascinating that he developed feelings, period xD).
I think Ulquiorra was initially attracted to Orihime because she is everything he isn't. She is kind and caring, strong and brave; but she is also weak at times, fearful, jealous, selfish...All in all, she is incredibly human. The exact opposite of Ulquiorra who, unlike the rest of his humanoid arrancar buddies, did not feel anything, whether good or bad. I kind of see them like magnets. If Ichigo and Rukia are like the 'yin and yang' who complete each other, then Orihime and Ulquiorra are magnets- total opposites that are inevitably attracted to each other (not necessarily in a romantic or sexual way).
Being with Orihime, Ulquiorra's heart was basically created. Not just the good: he felt anger or wrath (see him fight against Ichigo- he's definitely way too passionate about defeating him and making him 'despair' for it to be mere apathy), and he felt, in his own poetic words- lust. Sure, this is Ulquiorra we're talking about, so it's only natural that he'd only point out all the negative feelings he developed, but all one needs to do is see his final scene to know that there were positive feelings in there too.
So all in all, Orihime gave him a heart. That's no easy feat. I think that the sole fact that it was implied that Ulquiorra 'lusted' for Orihime means that the negative emotion was accompanied by something positive. Aside from every other feeling he discovered, he found something that even if it wasn't exactly 'love', it was damn similar. Ulquiorra was interested deeply in Orihime, and he, presumably, lusted for her; we don't have much solid proof, but the fact that he had a chance to kill her and chose not to (on the lame excuse that Aizen hadn't ordered it yet) points at the fact that he did care for her in some way. All these are perfect ingredients for him to be in love or well on his way to it. I'm definitely of the opinion that if Ulquiorra had lived, he would have fallen in love with her.
So I think that he was on his way. He was on the road to falling in love, without a doubt. I think that in the last moments of his life, as he became ash, he probably did feel it- love, that is. He might not have had time to fall in love 'properly' (lol, as if this was a procedure), but I do think that what he felt when he speaks of discovering the heart was love. Made all the more evident by his search for reassurance when he asks Orihime if she's scared of him; I think that was his way of asking if maybe, just maybe, she cared for him in the way that he had come to care for her. Not that this was his conscious train of thought when all this happened, I doubt Ulquiorra ever tried putting a name to his feelings, but still, that's my interpretation.
I really love the relationship between Ulquiorra and Orihime. They're really that romantic idea of 'night and day', and the way their relationship was developed and portrayed gives one hell of a layer to Bleach, beyond just having a bad guy kidnap a girl to move the plot forward. It's so sad that it had to end (but storywise, it was such a perfect, bittersweet ending T.T Curse you for being so amazing Kubo!)