I have a PS3 and a PS4 with the same PSN account. Given from the information released by Sony with one PSN account I can have up to two PS3s and one PS4. Am I able to play online simultaneously with both consoles? Is it also possible to play online against each other?
After some more, proper, research and actually trying it again, it seems like it is not possible to log in to the same PSN account on two different PS3's at the same time. It is possible to have a PS4 and PS3 online on the same account simultaneously, however.
As for playing online versus each other, I can't say for sure. We haven't actually tried this with games that supported it. There aren't many (if any?) games that support cross-generation (PS3 vs PS4) play. Playing PS3 vs PS3 should not work since the system won't allow you to log in with the same account on both machines at the same time.
IF you try to log in on a second PS3 if you are already logged in on another you will get a message on the logged in PS3 to say that another user has signed in with your ID on another console, and will subsequently log you out as they log in, thus you cannot be online with two PS3's at any one time.