I'm obviously lurking here, but I'll tell you what my OB said.
Based on my LMP, my due date would have been 4/16. Based on my early dating ultrasound, my due date is 4/30. At my last appointment, without me even asking, she said, "So your due date is based on that dating ultrasound, and since you had that, it WILL NOT change, no matter what."
At least she didn't tell you, no way, we're inducing you... so maybe that's a good sign? :)