Looks like the Left4Dead content coming out this fall will be free for both consoles.
Microsoft has tried to maintain a certain "market value" for all it's content on the XBLM. After all, people might be reluctant to pay for a Gears or Halo map pack if they just got their L4D maps for free. I can imagine how Valve convinced Microsoft to allow this sort of heretical price tag on the XBLM. "Go ahead and send the message that people should be buying our games on the PC and not the 360 if you really wanna charge for this stuff."
I'm glad as the game shipped a little light on the content side. Even by Valve's new "you'll only play a few maps anyway, just look at Counter Strike" ideology, the 4 campaigns are starting to wear thin.
I'll admit, I was hoping the 360 content wouldn't be free. I engaged in a some petty PC superiority posturing with a 360 L4D player awhile back. This news only goes to prove his point. But really, what does it matter who's right or wrong when we're speculating on the backroom dealings of game publishers? We all end up on the cover of Modern Jackass.