SEGA has launched a new website which confirms that a sequel to Yakuza will be released in Japan this year.
Ryu ga Gotoku, or Yakuza to Western gamers, hasn't made it to European shores just yet (scheduled for September 15), but the game's been around in Japan for quite some time. So long in fact, that a sequel is due out this December. You can find more information over on the new website.
There's nothing confirmed outside of Japan, but Yakuza is one of SEGA's premier new IPs, and assuming the first title does reasonable business in North America and Europe there's no reason why we won't see the sequel over here sometime in 2007.
To prepare yourself for the European release of Yakuza, SEGA has been releasing segments of a 40-minute live-action prequel movie over on the game's official website. Parts 1-3 are currently available, and it shouldn't be too long until part 4 is released.