Is downloading copyrighted music while you are a LimeWire pro user illegal?

NO.

Limewire Pro means you are paying for the media that you download. So that means it is not illegal. This is because you are technically BUYING the media that you want at home instead of going to the store.

If you have NOT paid for LimeWire, however, then it IS illegal. You are stealing the media. Downloading media from LimeWire for free is the same thing as robbing a store's CD's.