I was wondering the same as valrama, though the answer given raises two more questions:

-Can a pool 'hide' behind for example Tor, as in, change it's IP-address each time a valid block is broadcasted, and obtain a new Bitcoin address, too?

-So we have a pretty good idea of the hashing power of registered pools, but there's no insight in private (e.g. government backed) pools, or just a miner with a very large amount of hashing power (and money for mining hardware), both being able to change their Bitcoin address for each new coinbase transaction?

The 'concern' I'm having is that there's no way to find out if a miner obtains near 50% of the total hashing power. Though it's not likely to happen considering the costs, I'm curious if we'd be able to detect such a case, where a mining pool puts effort in remaining anonymous.