Hello! I compiled and linked Guile `master' (as of cb9d473112ac172a3d328bb029b5b550918d4262, dated 2009-03-09) with `--coverage', then run "make check" and collected the results with lcov(1):
http://www.fdn.fr/~lcourtes/software/guile/coverage/ The test suite covers more than 70% of the API. The good news is that there's room for improvement, but OTOH some parts of the API can't be easily tested (parts of `posix.c', `gethost', etc.). Thanks, Ludo'.