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" approximations.
 It's also printed in his "Notation, Notation, Notation" book.

From: Jeff Russell []
Sent: 19 August 2010 16:32
To: Game Development Algorithms
Subject: Re: [Algorithms] fast pow() for limited inputs

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.