They promised an Apr 30th delivery via a press release. If they don't arrive then, I expect some sort of refund or iTunes store credit.
You are certainly entitled to your refund....which I think you would see before you see itunes store credit.
So let's say you demand store credit or you will request a refund. Given the current popularity of this device, Apple will gladly refund your money and turn around and sell "your" unit to one of the thousands of folks impatiently waiting for a 3g unit.
By the way, do you believe Apple will give itunes credit to our international friends whose launch was delayed a whole month? I think the international community was initially promised units by April 30th.
Hopefully you will receive your unit on the 30th and this won't be an issue.