Visa for hong kong from mainland china?

Hi there, I have a business visa valid until Nov 2011, It is one entry which I used to come here, I wish to go to Hong Kong at the end of October and then re enter China for a few weeks before leaving = is that possible on the one visa? Do I need to get a new visa to go to HK? Or, if I don't, coming back to the mainland will there be any trouble?

Thanks for your help!