We'll have a beta 2 out shortly and the trial will be extended to 60 days. It is true that if you do not have a FB3 serial number, you will not be able to extend your trial beyond the 60 days. If you were to buy FB3, you would need to upgrade when FB4 is released.
I'm going to need 6 months to build what I want to build and see if it's going to work in FB4.
Products are looking good BTW despite the bugs in the betas. Adobe has borrowed a lot of excellent ideas from other IDEs including Delphi, my primary IDE.
I think that proof of concept should be at most 20% of the project time unless you are like me and the POC is almost the release version
If you are lucky, if it is like Flex3 the later beta's had a trail period rather than a beta period (60 days). That should be more than enough for making the decision on moving forward with a project. I am sure that if it arrives at a point where you want to use FB for a project and the release isn't ready Adobe will sort something out for you.