Re: [ANN] Ethereum: Welcome to the Beginning

So, now that we "have" sidechains (or at least, the project of how to build them), ethereum could be built as a sidechain of Bitcoin?

No, because Ethereum uses its own coin.
You can 2-way peg any coin to another, but that's not a sidechain.

http://www.reddit.com/r/ethereum/comments/2k6otf/implementing_ethereum_as_a_bitcoin_sidechain/clj59or?context=3

Two way pegged should not be considered sidechain.
Sidechain means a chain that depends on another chain (the main chain or a higher chain).

Bitcoin and Ethereum blockchains will be independant, none is sidechain to another.

Seems similar but it is not. You will be able to enter/exit instantly in both scenarios, but while you're in a bitcoin sidechain price will be pegged to bitcoin. That means if you buy 1btc in the sidechain and after you cashout you will have 1 bitcoin. But if you move to Ethereum you will (instantly) buy Ether at market price and then you will cashout at another market price.

In a sidechain you can't create new coins (not even with block mining reward), only transform coins from the main chain. In a 2-way pegged altcoin (like Ethereum or any other) you can.