As a speech-language pathologist, I agree with the previous poster who mentioned that sippy cups are particularly an issue for children who have any kind of oral-motor and/or speech issues.
I was a big sippy cup offender myself with my dd (now 7), because I didn't want the spills on the carpets, etc. But she never had any kind of problems with speech or oral-motor development. And that was before I knew about all the issues with sippy cups.
I might just mention, though, that for anyone who is interested, that it's actually sippy cups, and bottles, and pacifiers, too, that are culprits for speech and oral motor development, if they are used too long. Certainly, babies have a need to suck, and that's fine and dandy. I'm not aware of any magic number as far as when they need to stop, but I would just be aware as a parent, that bottles, sippies and pacifiers can all hinder speech and oral-motor development.
The reason is that with each of these items, the tongue pushes the nipple or spout against the roof of the mouth in a forward thrusting motion, and if done over a long period of time, this can cause the tongue to be carried too far forward in the mouth, so that a tongue thrust is developed. That can cause things like kids saying "th" instead of "s" ("thoap" instead of "soap"), for example.
I'll climb off my soapbox now

but just thought I'd throw my 2 cents worth in!!
Best wishes to all you Moms! Parenting isn't an easy job, and it isn't for cowards. I tell you, if it's not the sippy cups and pacifiers, it's the little seven year old attitudes!
