You can see the old (v0.6) sources in /var/tmp/portage/dev-lang/mono-2.8.1-r1/work/mono/mcs/class/Npgsql According to gentoo filosophy, that driver shouldn't be built with this package. the current version of this driver is 2.0.11 See also http://bugs.gentoo.org/343253
Well, mono is not being built against npgsql, it is simply installing a different version than the one provided from http://npgsql.projects.postgresql.org/ (like all the other major distributions are doing)
This is upstream: https://bugzilla.novell.com/show_bug.cgi?id=650180