The WWE.com makes an announcement that Christian quit:

WWE: Inside WWE > News > Christian quits WWE

WWE.com has learned that SmackDown Superstar Christian has quit World Wrestling Entertainment. Be sure to stay tuned to WWE.com as more information becomes available.

I have heard from some of my sources that  Christian's contract expired and he decided not to renew.  Also his bio has been taken down from WWE.com

To tell you the truth, I don't blame him one bit.  Christian was so over with his peeps and things were going good a few months ago, then all of a sudden, BAM!!!  He becomes a second class citizen.  I'm sure Christian will do very well on the wrestling circuit, he is a very talented wrestler.  I could see him doing very well in TNA and overseas.

Real Name:  William Jason Reso

Birthday:     November 30, 1973 

5 PM UPDATE

I am told by a reliable source that Christian is at Raw right now.  Nothing may be worked out, but he is there.