Hi
Yes, what we are talking about here is not whether the games on PSP and PS3 are region-free or not. You CAN play games from any region in any console without problems.
The question here was: if you buy a game from the US and your friend buys the same game in Europe and the game can be played online, will you and your friend be able to play together?
As far as I have seen, the answer to this question is: No, this does not work.
At least from my experience:
I know that, for instance a European version of GTA (PSP) cannot be played online with ppl who have a US version of the game.
I bought Motorstorm (PS3) in the US and played it online on my US PS3. Even though I was logged in with a European PSN account, I could only see US servers to play on.
Now, the only remaining question is: Was that because the game is a US version or because my PS3 is a US machine? I believe it's because of the game, but I will be able to confirm that 100% in a few months when I go back to Europe and can try to play my US Motorstorm on a German PS3. But I'm willing to bet that I will still only see US servers and so that it depends on where a game comes from.
That's a bit of a bummer, as I will apparently not be able to play this game with friends in Europe. This is definitely something that one should be aware of, I think, before buying a game with online-functionality.