I think that it was on the cypherpunks list that I learned of how PGP for the IBM PC, running under emulation on the Mac failed to produce good random numbers. The virtual PC clock proceeded forward by very predictable manner. Perhaps the details were different but the nature of the pitfall is clear. I did not notice that pitfall mentioned in RFC 1750. (Its the only hazard that I know of that they missed.) The only thing I can think of protecting against this is to do some simple checks against more obvious ways that virtual clocks might produce times. Low order bits should not always be zero. The differences between successive readings should not be constant. Two clock readings separated by a computation of known length should be within a factor of a few of the expected value. If not try again once or twice. Such tests are imperfect but I think that they would have noticed the virtual clock on the virtual PC. If they fail the program can require the user to enter the seed, with all that that entails.