Understand, but if you happen to have some hanging around doing nothing at zero cost
Well, for the newer Virtex-6 and Virtex-7 chips Xilinx had to release a new tool set, because the number of gates made their existing programming tools intolerably slow and impractical. With the 28nm process used on the latest Virtex-7's there's even a decent chance that they would be capable of competing with the ASICs that are (Avalon) and might be (BFL and post buy out bASIC) shipping.
The catch is though that for these chips we are talking about a $6,000 to $10,000 per chip cost, so there probably aren't too many of these just laying around waiting for people to check them out from work, appropriate them from a university lab, or filtering down into hobbyist hands for quite some time. There's also the matter of how much of a pain in the ass it would be to program these for mining seeing how long it took for simpler FPGAs to get configured for this purpose on any substantial scale.
It would be awesome if someone could so this as a proof of concept though, maybe someday SHA-2 hashing will be a benchmark FPGA manufacturers use to express the relative performance of their products. If this were the case in 2011-2012 the mining landscape would probably be very different, because someone might have seen a business case to run with high end FPGAs over GPUs and even low end FPGAs.