I'm pretty sure L4D will be a success just on PC sales alone. I can understand developers who were PC centric first favoring 360.
However, even when you're rolling in money you're in the business of making more, and this is simply noting that when a good multi-platform title can sell as well on PS3 as 360 why leave half the potential sales/profit on the table?
Assuming PS3 continues to grow install base well (which I'm pretty sure it will) and with ID aggressively aiming to get back into the engine middleware market with a new engine designed to be truly multi-platform, I think we'll see both Epic and Valve look to improve their abilities on that platform. Valve I think is the furthest away, as Source never seemed to take off as well as U3 engine for middleware sales, but I can't see them continuing to ignore PS3 is the sales are there to be had.
I wouldn't hold your breath though as it will likely be a slow transition. I bet they'll watch Bioshock sales on PS3 with interest - if it shows a strong PC/360 game can sell very well a year later on PS3 then I think the odds will shift from 50-50 to favorable odds for a PS3 version at some point mid to late 2009.
Try to be reasonable... its easier than you think...
