Katherine has not left Grey's. She is taking some time to both film a movie (Life as We Know It), and spend time with her newly adopted daughter. She will only be absent for about five to six weeks, and then is expected to return, as Executive Producer and Creator Shonda Rhimes said via Twitter.
http://tvwatch.people.com/2009/06/19/katherine-heigl-returning-to-greys-anatomy/ - People Magazine Article
http://www.cnn.com/2009/SHOWBIZ/TV/09/02/heigl.greys.anatomy.hiatus/index.html - CNN
http://twitter.com/shondarhimes/status/4949809869 - Shonda Rhimes on Twitter confirming Heigl is not gone.
http://www.eonline.com/uberblog/watch_with_kristin/b149412_which_one_of_new_kids_replacing.html - E! Online article