IF there is another AWB (which is definately not a given), it will PROBABLY go into effect on a given day, and any rifle that is not constructed by that point will be subject to the terms of the ban. They won't care whether or not you placed an order, if the gun is not assembled on that day and that time, it would be subject to the new law. However, it all depends on how any hypothetical law is written. Some versions of the 94 AWB had confiscatory language- that is, any AW, no matter when it was built, was illegal and had to be turned in. That sort of thing is still possible, of course, but significantly less likely than a ban of further manufacture.
What's interesting is that the Obama administration has always had renewal of the AWB as a plank in its platform, but until Holder opened his big mouth, they had been very unwilling to talk about it. Now that Holder has mentioned it, the Dem leadership in Congress has said they won't go for it. While none of this makes me complacent, it does mean that there is a very good possibility that they'll do nothing about it until after the midterm elections, and maybe not even until Obama's much-hoped-for second term.
Mike