Ours only have to wear shoes on the way to school and back, they take off their shoes all year round (not just a winter thing) once entering the classroom.
Not a big fan of the whole barefoot idea (think hygiene). But in the German Kindergarten the girls attended for a few months last year, every child had slippers and you changed into them in the morning so you wouldn't run around in potentially muddy boots inside. And in general, I really like the idea of wearing different things inside and out, seems to much cleaner to me.
Mind you, mine play sports in school barefoot, so the much cleaner option would be to have them put on their shoes to ENTER the classroom, since their bare feet are all muddy...
Daniela