BFL states that warranty for even end-of-life products is to be no less than 1 year from date of sale (this can be found in the FAQs). I believe that BFL has stated that a new product line is only when a re-design of the chip is done on a lower die size, so that is obviously a long time away from now. 65nm is still pretty new for a small company to make a custom chip on, and recouping the development costs will most likely take some time.
Once we stop manufacturing that particular product, we will not be able to warranty the product any longer, since we no longer have replacement stock.
I see nothing referring to changing die size. It's pretty clear. Once they stop manufacturing that model, all warranties revert to a 1 year warranty. It really is just a 1 year warranty.
What I mean is, I can see no reason for a total re-design of an ASIC product except for production technology advancement such as die size. Other than that, there can be a revision - such as a fix to a connector or whatever may come up, but given BFLs excellent history with backing their products, I am sure this is a non-issue when it comes to warranty support.
There are already smaller process sizes in use today, for those with deep enough pockets. Intel's Ivy Bridge chips use a 22nm process, vs BFL's 65nm. Remember that the size advantage saves space in two dimensions, such that halving the process size results in a quartering of the die area.
Another redesign possibility is on the logic level, designing the hashing engines to fit into a smaller die area, to scale to higher clock speeds, and most importantly, to consume less energy.
There are probably also other areas where the design can be improved upon, such as improved thermal coupling of the chip die to the chip package. Using a flip-chip design for instance.