According to the new Worlds 2009 schedule, a Weavile distribution will take place on Saturday, August 15th, the last day of Worlds this year. The Weavile will be available to English, Japanese, Spanish, French, German, and Italian Diamond, Pearl, and Platinum games and will most likely be given out via direct download like Lucario last year.
Speaking of the schedule, if you just want to go to Worlds for just one day, Saturday seems to be the best day to do so. Supreme Victors prereleases, the final TCG matches, the entire video game tournament, the VIP signings, Pokemon League, and Pokemon movie screenings will all take place then. Thursday is mainly for the Grinder, while most of Friday is dedicated to the meat of the TCG tournament.