According to Tom's Guide, Bethesda will be announcing The Elder Scrolls: Online in May, likely during E3. Due to the site's reliance on anonymous sources, we're chalking this one up as a big fat rumor, though one we wouldn't mind seeing come true. Tom's Guide, however, is very confident in the story.
They report that the MMO will take place a full millenium before the event of Skyrim, or sometime during the setting's "second era" when Tiber Septim was appointed emperor. Player's will choose between three factions, represented by a lion, a dragon, and a bird of prey. Not enough info to get excited over, but still significant if the rumor proves true.
It's no rumor that ZeniMax Media, owners of Bethesda and id Software, have been ramping up to release an MMO of some kind. In March 2011 they founded an MMO support facility in Ireland, despite having no announced MMO. In 2007, ZeniMax formed ZeniMax Online Studios to work on, "the development of cutting edge MMOG titles," but has yet to put out a single game.
Only time will tell! Would you be surprised to see an Elder Scrolls Online announcement so soon after Skyrim's release?