I wonder if this is EU-specific, or is this change global?
I think they announced this for Japan as well, and I suspect the new policy will at some point be in place every where.
Frankly, like other exploited features, I'm surprised they didn't do this earlier considering the widespread exploit of this feature to get free games. Heck, there are even dedicated threads all over the internet where people are blatantly game sharing.
The third monitor +DLC set-up (while extremely rare) does raise an interesting question. Until it's an issue I guess we wont know for sure, but I suspect there will be a solution.
I believe this policy change is not retro-active, so it will only impact PSN games and DLC purchased after the policy change.
That said, besides a 3rd monitor + DLC set-up, even if you have more than two PS3s it isn't an issue - all you have to do is activate the account you want to use on the PS3 you want to use - such that you could still have five or even a dozen PS3 with the same account on them, but only two can be activated at the same time. I seriously doubt anyone would ever need to use the same account on three PS3 at the same time, and even if someone can come up with some scenario, I doubt its one most would even consider.
In a sense, people could still exploit this by letting people continue to game share... but this makes it less convenient to exploit the system as the account holder now must make sure they keep track of who they give their account info to and to make sure they deactivate the account when they are done playing - or else the account holder may risk being locked out of their own games and content... and may have a hard time explaining themselves to Sony when they ask to have their account deactivated on a system they don't own.
Personally this is a good move as it doesn't appear to impact honest consumers (we'll have to wait to see how this impacts 3-monitor systems), as they can easily activate and deactivate their own systems and thus could have the same account on dozens of PS3s if need be, but it makes it much harder for those exploiting the system to get free games.