A bunch of loan-words from Latin and Greek used to be pluralized with "a," since that's the way it was done in those languages (for some words).
But we can no longer make up our minds about whether media is singular or plural. So also with bacteria (when was the last time you heard someone refer to a bacterium?). And lots of people treat the Greek loan criteria as singular (though it used to be the plural of criterion), even to the extent that they've invented a new, English-friendly plural for it: criterias.
It used to be that you could count on the NY Times to be futzy about stuff like this, but it looks like they've given up:
[The scientists] hope that a better understanding of how Arctic climate has varied over the millenniums will help them project the implications of the region's recent warming trend . . . Read it all (registration required).
Millenniums? I guess you can't say millennia without infringing Mazda's trademark.
Not that I'm complaining. I don't go around saying stadia or auditoria, though that's they way stadium and auditorium would be pluralized in Latin. And even though the Romans had a Colosseum, I don't mind going to the Greensboro Coliseum.
UPDATE: media (singular) has an English-ized plural, too: medias.
Google is a great resource for linguistic research into English as she is spoke!