Haswell is only a few months away and is a significant upgrade to Ivy Bridge. If Apple went with Ivy Bridge now, they'd lose out to competitors who are already ahead of them. Does Apple want to be behind the curve? No, they want to be ahead of it. Therefore, Apple will release with Haswell after this summer, but they will get exclusive early access to Haswell, so they will get their laptops out before the official launch.
The last MBP update was only 185 days ago. Most updates are >300 days. Therefore, we have a long ways to go and Apple certainly won't release in the next few weeks. Also, since this is a new design under a new CEO, they want to make sure they get it right. Consider all these factors, and you'll realize that the next MBP update won't be here for a long time.
Another thing to keep in mind is that Haswell is expected to come with built-in Thunderbolt technology. Coincidence? Does Apple do ANYTHING that is a coincidence?
I have read that Haswell is 11-14 months away, pending delays.