I believe it's out-of-stock or the stock haven't come in yet for all merchants. So, you have to wait.

It's not just IS, but the optics have been improved over the old version. I guess if the the difference in cost between XT body only and the XT kit isn't that much, then just go ahead and get the kit and wait for the 18-55IS. Then, use the original lens as a paper weight afterwards. Or you may consider getting a 50/1.8 (cheap, but great value) to start while you wait.