Out of curiosity, why does it take 120 confirmations, vs. slush only takes 100 confirmations?
If you Solo mine, and actually find a block... the 50 Bitcoin reward appears as a "Generated" transaction... as opposed to a "Sent" or "Received" transaction.
The Bitcoin protocol requires 120 confirmations before "Generated" coins can be spent. Since the pool Generates the coins, eleuthria is just making you wait the same amount of time that he has to wait before you can get your reward. I have no idea why any other pool would not use that same 120 confirmations.
The 120 confirmations are required to make absolutely sure that the block that was found is valid.
Now, let me take this time to remind everyone that you can skip all the waiting if you adjust your donation to 2.5% or more.
Not only do you help eleuthria pay for the servers needed to run the pool, but when a block is found, your estimated rewards goes instantly to your confirmed rewards. Even if a block is invalid, you still get paid.