Hmm, interesting situation. I do know of people who actually changed their travel plans in order to enter the day before the one listed on the visa, just to be safe. I'm really not sure how you'll be received at immigration. Moreover, you may not be allowed to check-in by your originating airline, depending on where you are flying to China from. If you have time, consider getting a new visa. If you will be doing another future trip to China after this one, definitely get a new visa.

On a sidebar note (that's not your particular issue right now), confusion about Chinese visa terminology sometimes comes up on the forums, so I'll add the following for those readers. Generally terms generally fall under one of two types depending on visa type and where it was issued:

1) If the visa has "valid until" than that is a hard-wired expiration date, and you need to be out of China by that date, or get an official extension. After that, a new visa must be obtained. This is pretty easy for most people to understand and many countries do their visas this way.

2) If the visa has "enter before" than that is the last day (actually the day before to be on the safe side) that you may enter, and your duration of stay will be whatever the terms of your visa states. Example: My current visa has an "enter before April 3, 2010" on it, with a duration of stay of 90 days each entry. I entered China last on February 26, 2010, and I must leave the country by May 27, 2010. If I left China this month, I could re-enter on April 2, and reset the clock another 90 days from then. Then it's leave the country and new visa time. (Many of the visas, especially the 12-month multiple entries, are being issued to US citizens in the US using this terminology.)