Besides that pesky thing known as "ethics", what's to stop the following scenario: 
 
1) Buy Portal for PS3 
2) Unlock free Steam version 
3) Immediately resell physical game used to retailer before the amount of people doing this makes the game essentially worthless 
4) Profit?

My assumption is that every copy will only be good for one steam unlock. So you could do that, but the impact wouldn't be much worse than just reselling it when you beat it.

" My assumption is that every copy will only be good for one steam unlock. So you could do that, but the impact wouldn't be much worse than just reselling it when you beat it. "

I'm keeping my PS3 copy for on-the-couch co-op.