as we all know that bitcoin was designed to be a decentralized currency. Yet people have pointed out that 50%+ computing power is owned by only few mining pools.
The statement "the majority of X is owned by the top N largest owners of X" is true of any X and some (usually small) number N, and does not mean that X is centralised.
It really hits me hard when people still claim bitcoin as a decentralized currency, especially after yesterday's unfortunate Mtgox incident. Yes, Mtgox surely did a bad coding job. But please really think about who were the attackers?
Hanlon's razor: Never attribute to malice that which is adequately explained by stupidity.
In order for such faked transactions getting included in the blockchain fast enough and "lucky" enough, I assume it must have something to do with those "centralized" groups of people, at least.
There were no faked transactions. Only real transactions that Mt. Gox believed it never sent, due to the combination of their incompetently engineered software and non-existent audit process.