Claims are coming in from all over the 'net that running Windows Media Player in the background while playing World of Warcraft shortens load times.
This phenomena was originally reported on WoW's European forums back in November, but the quirk recently jumped the pond and reports of players benefiting from this oddity are now appearing on the American forums as well.
After noticing the folks at WoW Insider offering their own anecdotal evidence for the viability of these claims, I attempted to test the method myself. By opening Windows Media Player, leaving it idle, minimizing it, then playing WoW, I did in fact notice shorter loading times, specifically in high-traffic sections of the game.
Update: We just received word from Blizzard that they are aware of the "issue," and are looking into it
Media Player boosts WoW performance [WoW Insider]