Andy,
The key is
flash duration NOT shutter speed. Camera on bulb in a dark room so the shutter is open. Even Stevies D100 can cope with that. As long as it is recognised that us mere mortals cannot get to flash durations greater than 1/50,000 sec then there are still lots of subjects that we can do. Rifle bullets cutting cards does require some pretty specialst kit in more ways than the flash dept alone though, I agree.
1 million/sec flash now that would be serious money.
Probably worth a mention on the flash front. Studio units are no good for this as you cannot get anywhere near the short burst duration that a decent camera flash on reduced power will give.
Don