It would be nice if there'd be a variable like "DEVELOPMENT_STATUS" in the ebuild files so one could e.g. specify a certain level of stability for production vs. testing machines which newly installed ports shouldn't fall below. For some reason, it might be useful to have beta and unstable software present (for the daring users), while installed packages would be stable by default (two different branches in the same port). Furthermore, sometimes it's difficult to determine the actual of status when reading the ebuild file - there is no documentation about that yet. It's only the port's maintainer's decision what version is stable enough to be set free into everybodies' systems.
We now have KEYWORDS functionality for this. To get experimental packages, add ACCEPT_KEYWORDS="~x86" (for x86 systems) to /etc/make.conf.