A picture turned up on the Internet in recent days that suggests a new baby dragon whelp will join the pack. This one is appears to be a Frost Wyrm Whelpling. Those of you who've played Warcraft III will remember the mighty undead Frost Wyrms who hailed from Northrend, the continent featured in the next expansion. So I'm betting that this little fella will make his debut in Wrath of the Lich King, which is expected to hit stores in late 2008 or early 2009.
Further speculation suggests that this could be the special Collector's Edition pet. It would follow logic, as The Burning Crusade Collector's Edition gave us the Netherwhelp, a creature indigenous to Outland.
UPDATE: As of July 18, 2009 it's confirmed!