The World of Warcraft virtual world, which now has more than 1.5 million users, has experienced extensive downtime in the past 24 hours following a system upgrade for a content patch. The main web site is now back online, and the game's operator, Blizzard Entertainment, currently lists all server "realms" as being online But WoW forum postings indicate that many users continued to experience problems through midday Wednesday.

A dynamically updating chart of World of Warcraft's web site performance is available here.

Blizzard announced last week that World of Warcraft has more than 1.5 million subscribers, with as many as 500,000 users playing simultaneously. The game's infrastructure has peridically struggled to keep pace, suffering a series of outages in January. With monthly subscription plans ranging from $12.99 to $14.99, a user base of 1.5 million would generate revenue of at least $19.5 million per month - or $26,198 an hour. While MMORPG credits are often measured in extra time rather than cash refunds, that's a substantial "cost of downtime." Nonetheless, the performance problems seem to have had little impact on the phenomenal growth of the game.

World of Warcraft is a massively multiplayer online role-playing game (MMORPG) based on Blizzard's Warcraft series depicting a war between four unique races (Orcs, Humans, Night Elves and Undead) each armed with magical abilities and distinctive weapons.