I don't think this is a big deal. It is within the same company so it seems normal. I know managers in my company almost always give each other a call to get their take on the employee before someone is promoted in my company. I would just give your boss a heads up so he isn't caught of guard.
Internally I've always had hiring managers talk to my current manager before moving positions. I didn't think it was common practice for managers outside the company to ask your current manager for feedback anymore. I'm not saying it's anything to worry about, I've just never heard of it happening.