Tianhe-1A hashing power, very accurately IMO has been estimated as 850 Ghps. Assuming the rest of top 500 supercomputers have the same TFLOPS/Ghps ratio than bitcoin needs to hit 8Thps to dwarf all top 500 supercomputers combined. We are more than halfway there already. Another difficulty increase or two and 50% attack with supercomputers would require all those babies in top 500 list combined. Moreover while they configure them to do something bitcoin network would double once more.