As nice as IS would be, I won't pay $1500+ for that.
If all 135L's were that cost, you'd see a lot less people with them. The $700~800 thing as they are is why it's so good. If you made it double the cost, it quickly isn't quite as awesome enough to warrant hurrying up and buying it. While it's not the same, the 85L comes to mind, excellent lens, but it's cost is more the put off than anything else for most people.
I'm happy they will release an IS version of the 135L. They should do it for all their lenses frankly.
It will drive down prices on non-IS copies, so I can get them even cheaper!