I noticed "go" USE flag was masked on gcc:4.6, the problem is that I
just compiled it a week ago with USE="-go"... then, I would like to know
if there is a way to prevent it from being rebuild again :| (It will
take some time in my currently running system but on other machines I
maintain it will take hours)

For the future, wouldn't be possible (probably on a new eapi) to
indicate in some way to the package manager that:
(masked USE flag) is equivalent to (-USE flag) or (+USE flag)?

Maybe some indication in profiles masking file like:
$(category)/$(package) go (-go) -> it would indicate that, if version to
be rebuild was compiled with "-go", no rebuilding is really needed

What do you think?

Thanks a lot