Shu Takumi, the creator the Ace Attorney series, mentioned in an interview with Famitsu that Professor Layton vs. Ace Attorney is in its final stages of development! I'm betting we can expect a trailer at this year’s TGS and even a release date (for Japan).
Takumi has been very dedicated to the project since its announcement over 2 years ago, but it seems it's going to be worth the wait! Now all we need is a localization confirmation for the states!
Anyways, with Shu Takumi almost done with PL vs. AA he'll most likely be directing all his attention to Ace Attorney 5 now.