Professor Layton's London Life, a bonus RPG, will be unlocked from the start for the North American version.
http://www.nintendoworldreport.com/news/27623
Professor Layton and the Last Specter, the fourth in the series and last on DS, is coming out in North America on October 17.
The game, which came out in Japan in late 2009, is actually a prequel, kicking off a new trilogy of games that detail the origins of Layton and Luke's relationship.
In Japan, the game also featured Professor Layton's London Life, a 100-hour RPG created by Mother 3 developers Brownie Brown. This will be unlocked from the start in the North American version.
The game's story is followed up with Professor Layton and the Eternal Diva, a film that is being brought over to North America in November.