Are you looking for a creative way to "recycle" the bottles when the parents/babies are finished with them? Do they have to be practical to being a former bottle, or can it be for pretty much anything?
My first thought is to decorate them with paint, fabrics, beads, or whatever, and use them for storage for small things like craft items (glitter, beads...basically the stuff you'd decorate them with LOL) or in the garage for nails, bolts, and stuff like that. People could either use them whole and have the nipple serve as a lid, or they could cut the tops off and just have it be more like an open cup.
I like the birdfeeder idea, I think that would be a really fun preschool project, and show the kiddos a great way to use their old bottles to feed other creatures. Cute!
I found a small reference to using them as storage for both paint and paint brushes.
Here are links to a few others:
http://www.recyclethis.co.uk/20070706/how-can-i-reuse-or-recycle-babies-bottles
http://www.associatedcontent.com/article/13882/soda_bottle_crafts_creative_ways_to.html
http://crafts.kaboose.com/holidays/earth-day/bottles-jars-cans-crafts.html
http://www.craftbits.com/viewProject.do?projectID=2097
http://www.dotcomwomen.com/crafts/valentines/message-bottle.shtml
Some of these are for regular water bottles, but the thing is, I can easily see a baby bottle being used in place of a water bottle. You're right that there aren't a lot of sites out there specifically targeted to using bottles for crafty things, so you might want to look at projects that use baby food jars and water bottles to see how you could modify them for a baby bottle (those things come in so many sizes, the possibilities are endless!).
I've never met a craft I didn't like
but I've also learned that when I don't have everything on hand or when the things they suggest are out of my budget, looking around the house for "found items" makes it even more fun.