According to the official Grey's Anatomy website, the Season Five premiere will be on Thursday, September 25. At the present time, there is no other information on the premiere, and most likely there won't be until at least the end of next month. Here's the neat little synopsis that ABC wrote, however:

The doctors of Seattle Grace Hospital deal with life-or-death consequences on a daily basis. It's in one another that they find comfort, friendship and, at times, more than friendship.