if use mysql && use postgres; then eerror "Unfortunatly you can't have both MySQL and PostgreSQL enabled at the same time." eerror "You have to remove either 'mysql' or 'postgres' from your USE flags before emerging this." This is the wrong approach. In portage the standard is to choose one over the other when both USE flags are specified. In the case of nagios-core, you should decide which one makes most sense as a default (probably mysql, I'd guess) and use that if both USE flags are specified.
Quick question: which one? The database wars are almost as common as the distribution wars...
Prioritized mysql over postgresql. Change included in nagios-core-1.2-r1, now in portage. Thanks Aron.