Re-entry to China with a single-entry Z visa but a processed work permit

Hello,

I had a single entry Z visa to enter China in February, but now I have a current Foreign Expert Certificate which is valid until Feb 28, 2012. I need to travel outside of China to the US in August, but plan to return. Do I need to get a new entry visa or will my expert certificate be valid to re-enter China. When I applied for my visa, I was told I only needed a single entry even though I would be traveling in and out of China during the year, and I am worried about not being able to return to China for the upcoming school year. Thank you for your help with this matter.