More rumours abound
Furthermore, theres a chance this could be Monkey Island 5. With LucasArts becoming desperate, its likely that they could have sold the licence to someone who actually wants to make adventure games.
Mixnmojo
It could also be Grim Fandango 2, but thats not too likely either, as a) they might not want to do it without Tim and b) they might not want to continue on the story as Grim Fandangos ending pretty much summed everything up. So by the powers of deduction Monkey Island 5 is a fairly safe bet. Fingers crossed.
Well keep you posted on any further developments, but for now visit TellTales excellent web-site, and perhaps read this recent Q&A about the company.
Update: Discuss all your theories here.