GoPets, an online world established in South Korea in 2004, lets users explore, play casual games, and create and customize virtual pets. Now, Microsoft Casual Games has announced it will partner with GoPets to launch the game on Windows Live Messenger, integrating the virtual world with its instant messaging service.

The service is now available via IM to Messenger users in the U.S., Australia, Japan and Korea-- but Microsoft says it'll expand it to more than 30 countries by the end of 2007.

“GoPets is an online community where anyone can adopt a pet, customize it to their liking and interact with other pet owners anywhere in the GoPets virtual world at any time of day,” said Erik Bethke, CEO and founder of GoPets Ltd. “With direct access through Windows Live Messenger, it will be even easier for pet fans to make friends all over the world through common interests and shared experiences, such as chatting and playing games on the Windows Live Messenger and GoPets platforms.”

GoPets' basic membership is free, with special access for premium membership, and goods and currency-- "gold shells"-- for sale. Currently in an open beta phase in fifteen languages and eight territories around the world, GoPets recently surpassed 640,000 registered users, but that number is likely to grow, since Windows Live Messenger's userbase is pegged at nearly 260 million active accounts worldwide.

Erik Bethke, CEO of GoPets, recently spoke at the Casual Connect gaming conference in Seattle, on his online worlds philosophy. He'd rather use the word "citizen" than "user," for one.

"I believe that when we sell something with virtual currency, people believe it's theirs," Bethke said. "They should be able to buy, sell, trade and everything."