Hm, not sure, Bryan. The original (1926) edition of Fowler's Modern English Usage says that 'forum' is one of those words ending in '-um', originally Latin second declension neuter nouns, which are sufficiently naturalized into English to have the plural '-ums' rather than '-a'; others include 'album', 'asylum', 'harmonium', 'laburnum' and 'premium'. Some of what Fowler calls 'learned' words still take '-a': 'bacterium', 'curriculum' 'memorandum' , 'stratum' etc. In a third group are words that can take either plural: Fowler mentions, amongst others, 'aquarium', 'compendium', 'interregnum' 'medium' ('-ms in spiritualism,' says F.), 'rostrum', 'spectrum' and 'ultimatum' ('-ms better', says F.)