First of all, no offense to anyone, but I find this topic really annoying and stupid.
It's not only carpet, if you're not doing windmill right you probably are doing lots of stuff wrong, ie stabbing, collapsing, etc. Don't just blame everything on carpet, sure it's good to practice on regular floors but it's pretty dumb to blame it on carpet just because you're having a problem milling...I'm practicing on carpet and it's going fine, I almost did 2 spins the other day.
It's also benefitial when you move on to regular floors from practicing on carpet because it makes you lot faster and easier to do them. I hated carpet (and still do), but sometimes you gotta stop blaming other shit and practice harder..that's what I learned.
Think of it this way, at least you won't get banged up as much as you'd get banged up if you're practicing on a hardass floor..
Also go here and see other people's comments about learning mills on carpets.
Windmill on carpet
Thanks for posting!