Corrected entry: At the Orpheum finale, the chipmunks tear off their concert clothes, revealing them to be nude underneath. After that, they are captured by Ian and trapped in a cat carrier. Later, when they reunite with Dave, they are in their regular clothes. It's obvious that they didn't bring their regular clothes to the Orpheum and there's no way they could get them when they're trapped in a cat carrier or when they escaped.
Brad
7th Apr 2008
Correction: They got the clothes from the pile of dolls that were next to them when they were put down.