I've talked about this a few times on HN, but this is a huge pet peeve of mine. It's armchair litigating. Rovio could absolutely file a case against this developer. How far it would go would depend largely on the judge in the case. You can bet that Rovio would, if they decided to, file with a court that historically rules in favor of their position. This would mean that, at a minimum, this developer would have to retain counsel and pay the thousands of dollars it would cost to get the case dismissed (not a slow process). There would be depositions, discovery, inquiry, and a slew of other bullshit that comes along with being sued.
You see, this is how the game is played. It's not a matter of who is right or wrong. It's not even a matter of what the rules say. It's a matter of who has the bigger bank role to pay for attorneys that are good at playing the game.
I know because I've been in this scenario. Prior to the plaintiff filing suit against us, everyone said the same things:
"They don't have a case!"
"This is baseless, it will be thrown out!"
"What a stupid premise, they'll get laughed out of court!"
This was not from one or two people, and not from laymen. This was from virtually every attorney I talked to (6 of them). We're now 3 years in to the suit and the judge seems content to extend the trial date in to the year 2012. The courts are in the business of having people in court. Never forget it.