Chinese language site WeiPhone notes (here's the English translation) that customers can have their iPhones jailbroken by the store's staff, who will also trim SIM cards to fit the iPhone 4's micro-SIM tray. Additionally, some are even advertising that they'll install apps for their customers after the jailbreak is complete.
Note that the iPhone 4 is not yet officially available for sale in China, so we assume that CU staff are jailbreaking older models.
As The Mac Observer notes, Apple has not yet commented on the practice. Any comment from the company should be a good one.
[Via The Mac Observer]