Valve's doing fine as is.
Their focus should continue to be their own in-house online distro services and their PC IPs second. If they want to expand to allow their games to reach the core gamers who are only on PS3, great. But it's still an issue of diverting developer resources into a PS3 exclusive development team. The costs in man hours would only be offset if they projected enough of those PS3 only gamers would jump at the opportunity to buy their games.
Personally, I'm inclined to believe that it's a smaller number than some might think considering that a lot of PS3 gamers either have an Xbox or PC with a decent VGA card in it. Valve games are pretty forgiving when it comes to PC specs after all.
If you like Valve games that much, you probably already have a game capable PC. If not a $199 Xbox will do.