My 17 mo. old just started doing this recently.....it is making us scratch our heads!
The pediatrician says all little ones have sleep/wake cycles, so it's normal. Usually however, they should be able to comfort themselves back to sleep on their own at that age (obviously really young infants need a feeding!)
We have just tried a good sized teddy bear --check every day for ripped seams etc... I am very paranoid--- in the crib and it is working beautifully. Turns out he was just a little lonely. He still wakes as usual, but talks to bear-bear quietly till he nods off again! NO MORE SCREAMING!!!!!!! :-)
Hope this helps!
Pumpingiron: Mine has been waking in the middle of the night, too, and he's 18 months old. But I've always kept a few toys in the crib with him (his favorite lovey, a book, usually another small stuffed animal). In the middle of the night I'll hear him talking or singing. He never cries, and like you said he knows how to comfort himself and fall back asleep. He's a thumb-sucker, so that probably helps.
I roll over & pop a nipple in her mouth. Although, I am learning to lay for a while with my hand on her first, and see if that comforts her.
Overall, I look at it this way: I am 26 years old, and I often do not sleep through the night. If I wake up hungry or thirsty or having to pee, I take care of my needs. I figure I owe the same respect to my child.
my daughters were in seperate rooms and the my youngest would scream at bedtime and wake up every now and then in the middle of night. so i moved them both into the same room to see what would happen and now they both sleep without problem. i guess my youngest just wanted a buddy in there with her.
What did you do with crying babies in the night?
Although, I am learning to lay for a while with my hand on her first, and see if that comforts her. 
