Two authors (Adam Back, Greg Maxwell) of the Blockstream white paper on Bitcoin sidechains answered questions in a Reddit "Ask Me Anything" (AMA). Here are some answers from the AMA :
To implement sidechains in Bitcoin, it appears a "soft fork" may be necessary.
Sidechains (from Blockstream) are not treechains. The two are "orthogonal". Treechains could be implemented as a sidechain.
You can't readily transfer the non-Bitcoin asset to the bitcoin blockchain without a protocol similar to "colored coins".
Blockstream aims to have "open development" with code available at https://github.com/blockstream.
Blockstream seems to employ two (or three) of Bitcoin's seven current committers. These Blockstream-employed committers do not see this as a problem due to employment contract items and the overall consensual nature of Bitcoin.
Ethereum can be made to work with as a sidechain to Bitcoin.
An altcoin may ship with an early sidechain implementation before Bitcoin has a chance to have a community debate and (maybe) do a soft fork to enable sidechains.