It's either one of these possibilities on what exactly is happening with The Undertaker;
1- They decided to move Undertaker to RAW without the draft and so Undertaker comes back on RAW after a month or something.
2- Undertaker has needed surgery for a very long time, he might have taken a 6 month leave from WWE to repair his injured body and then he might return back in late September or around early 2009.
3- Undertaker is having a gimmick change from his old and stale "deadman" gimmick that has been active for the last four years. We might as well see the American badass returning on July 4th 2008 or something similar to that.
4- Undertaker has left WWE for some other reasons, and is going to make his classic "long awaited" return at SummerSlam, Survivor Series, The Royal Rumble or maybe in his hometown at WrestleMania 25.