Is it possible to determine Bitcoin ownership distribution? Could someone guess on the most likely current Bitcoin ownership distribution? Are most bitcoins still owned by (early?) mining enthusiasts who are waiting for the price to go even higher?
No, it's not definitively possible to determine ownership of bitcoins.

It may be possible to identify miner reward addresses that are inactively used, but accumulated regularly at one point. Search for addresses with a large balance, the outputs of which have never been spent or have a large amount of unspent outputs.

It may be possible to find patterns in early mining, up until pooled mining came into existence. Search for blocks relayed by the same IP address, but at irregular intervals. This may indicate solo miners who "tried it out" but lost their keys or are in fact sitting on their wallets. This may only be possible through Blockchain.info's records, because the relay address is not preserved in the blockchain.