According to a Telltale press release, they will be announcing a new series at E3. Whether it is a new intellectual property designed by Telltale, or another licensed property is not known, but The International House of Mojo got the word straight from Telltale's Emily Morganti that the announcement will not be Sam & Max Season Three but is an entirely new series. Yay!
UPDATE (5/14/09): Destructoid says that a source has told them that LucasArts will be revealing plans to resurrect a classic franchise at E3. jp-30 mentioned in the comments at the mixnmojo article that Dan Connors said that he was in talks with LucasArts about licensing one of their properties, and that he would like to do an episodic game based on the Day of the Tentacle characters. Also, he pointed out that LucasArts renewed their Maniac Mansion (the first game in series that preceded the Day of the Tentacle) trademark in October. It's probably not very likely that these two announcements coincide, but I would love an episodic Maniac Mansion game! The original was the first adventure game I ever finished. ^_^