You might refer to our mails this spring. We (Bruno and I) are implementing a 'numeric adaptor' library, to be optionally used in our generic geometry library, but developed completely separately.
It is currently in the Boost Sandbox, https://svn.boost.org/svn/boost/sandbox/numeric_adaptor/
It provides interfaces to GMP and CLN (another big number library), which can be used the same way as normal (IEEE) integers / doubles can be used. We recently changed and simplified the design. It now works more or less as Joel mailed today. It is template based, no virtual functions here. So Joel's sample would become:
double x; boost::numeric_adaptor::gmp_value_type y; x = abs(x); <-- call the IEEE implementation y = abs(y); <-- call the GMP implementation
Also don't forget that Boost.Math has simple bindings for NTL and MPFR (the latter using GMP internally) that allow these to be used as any other floating point type would be (including std lib transcendental functions etc). Cheers, John.