In a new interview with Canada's The Magazine, Tom Felton and Bonnie Wright say that they will attend the opening of the Wizarding World of Harry Potter theme park at Universal Studious Island of Adventure in May 2010.
This answer was said by Bonnie in a questionable tone. The opening could very well change in the event of construction delays.
Also keep in mind that earlier reports have pointed to an April 2010 opening.
Thanks to Snitch Seeker for the tip!