I don't know how much accuracy you are really after,
but Jim Blinn had a little article called "Floating-Point Tricks" that was in
IEEE CG&A Jul/Aug 1997, which has some "log2" and "exp2"
It's also printed in his "Notation, Notation,
Thanks Fabian, good info there.
I'll check out minimax
polynomials, and as both you and Simon pointed out base 2 log/exp would probably
make more sense.