Hi
If she's quickly comforted by you going in and giving her the soother, I would think that would be what she associates with sleep. If she falls asleep that way in the first place, then I would bet that when she wakes during the light sleep stage (around midnight) she needs it to fall back asleep. If you're not ready to get rid of it, you may just have to bear with it until she is able of putting it in herself. When she is able to do this you could put a few in her crib at night (that's what my SIL does) and it seems to work. Would a nightlite help her find it? I know you say that the paci wasn't a prob before with sleep, but I think Tracy's book says somewhere that they become more dependent on these props as they get older.
It doesn't sound like it's any kind of pain (teething, gas, etc) since she is so easy to resettle.
Not that this will help you with your lack of sleep but if you look at the bright side, at least she's not a thumbsucker. I'm constantly worried that our lo won't give up his thumb ever.....at least when you're ready you can wean her off the paci whereas we can't take his thumb away!
I hope someone else can offer you some more help!
Zelka