Is there any possibilities that some people/organization could become a "central bank" in Bitcoin world by having authority to change Bitcoin's codes?
Not long ago, Gavin decided that dust transactions should be eliminated and nothing with an output smaller than 5340 satoshi should be relayed over the network. Regardless of whether or not this was a good idea (in hindsight, I think it was), a lot of people hated the idea and were quite vocal about it. In spite of the opposition, Gavin went ahead and added the dust transaction limit (along with lower fees) to 0.8.2 and released it. So far this sounds like "central authority" right?
Now let's fast-forward to yesterday. 0.8.3 is the current version, most (if not all) full nodes are running bitcoin-qt or bitcoind. You'd think dust transactions are a thing of the past, right? If that were the case, if there was a "central authority" controlling Bitcoin, I couldn't have possibly received a 500 satoshi transaction yesterday (which, at the time of this writing, has 219 confirmations) because that transaction would never have been relayed to miners.
The developers (and maybe the foundation) do have the power to make certain high-level decisions, but it's up to the people using Bitcoin and especially those who run full nodes and/or mining pools to decide whether to accept those decisions.
edit: The first paragraph is, of course, an oversimplification. Gavin did include a way for you (as a user of bitcoind or bitcoin-qt) to change those default limtis and fees for your own node, with a warning that nonstandard transactions might not be relayed.. but it wasn't expected (especially with the warning) that many people would lower them. The fact still remains that such nonstandard transactions are still being relayed, meaning some percentage of the network including at least one mining pool either lowered that default limit or never upgraded to 0.8.2, either because they opposed the change or because they just haven't been keeping up with the latest release. Considering the 0.8 hardfork issue I doubt many mining pools aren't keeping up with the latest release.