Ron Gilbert's joke was not a joke at all. Return to Monkey Island is a new entry in the Monkey Island adventure game series and will be released this year. The first trailer for the game can be seen here or at the top of the post.
Return to Monkey Island is a follow-up to The Secret of Monkey Island and Monkey Island 2: LeChuck's Revenge. Guybrush Threepwood is back, and he will once again be voiced by Dominic Armato.
The trailer shows that Return to Monkey Island is being made by Terrible Toybox, the developer of Thimbleweed Park, in collaboration with Disney-owned Lucasfilm Games, which was involved in the first four Monkey Island games under the LucasArts banner. The art director for the game is Rex Crowle, who was involved with the design of some good-looking games like Tearaway and LittleBigPlanet.
We don't know much else yet, so you might want to check the website for future updates. Gilbert wrote a post titled "If I Made Another Monkey Island" in 2013. He promised that if he ever made another Monkey Island game, he would announce it on April 1st.