Good news: According to the wikipedia page the third season (ARIA the ORIGINATION) will begin broadcasting in Japan in a couple of days. Yay!
Bad news: I can't seem to find any (legitimate) commercial releases of any of the seasons or the OVA. I really want to buy this one, preferably from a legitimate Australian licensee, failing that an American licensee. Grumble.
EDIT 6/1/08: For a much better review of the manga (and much of it still applicable to the anime) check out this article from The Comics Journal.