Incentive and 51% attack White paper:

"The incentive may help encourage nodes to stay honest. If a greedy attacker is able to assemble more CPU power than all the honest nodes, he would have to choose between using it to defraud people by stealing back his payments, or using it to generate new coins. He ought to find it more profitable to play by the rules, such rules that favour him with more new coins than everyone else combined, than to undermine the system and the validity of his own wealth."

Still, I've read bitcoin community is concerned with 51% attack:

http://blogs.wsj.com/moneybeat/2014/06/16/bitbeat-a-51-attack-what-is-it-and-could-it-happen/

Actual bitcoin implementation does or does not have incentive system as per whitepaper to discourage 51% attack?
Bitcoin has a huge incentive system to discourage a 51% attack. Bitcoin mining with CPUs is impractical. You have to use dedicated hardware that can only be used to mine Bitcoin and other coins that use the exact same algorithm.

So to acquire enough hashing power to launch a 51% attack on Bitcoin, you can't just rent computing power or buy CPUs. You have to invest in Bitcoin mining equipment. If you manage to harm Bitcoin significantly, you're just turning your expensive mining hardware into so many doorstops.