a simple taxonomy would do (sort of). however, i've run into the same problem with the "DB support" (mysql vs. pgsql vs. xxx) taxonomy term i want to apply to release nodes and/or project nodes: the DB support only makes sense for certain kinds of projects (modules), not others (themes [god help us if there's mysql-specific code in some theme], translations, etc). likewise, the "php version" vocabulary would be meaningless (and confusing) for translations at least, and probably themes as well.