Level-5's CEO, Akihiro Hino, has announced that the company are currently developing a sixth entry into Professor Layton series. Hino announced it through his Twitter account and it was then translated by Andriasing.
The series is already two games ahead in Japan, with Western gamers still waiting for the fourth and fifth titles, a DS title and a 3DS title respectively. Hino-san did not say whether the sixth instalment will be heading to the DS or 3DS, but presumably it will be on the latter.
Hino-san also tweeted about the success of the series and how well it's sold. It's taken over four years, but apparently Professor Layton and The Curious Village has finally crossed the one million sales mark in Japan.