So I heard that games on Steam can be preloaded before the release date so that they can be played as soon as the game comes out.

Is this true?

Also any ideas as to when Dishonored will be available for preloading on Steam?

Not all games can do this, only a few. But seeing this is a rather large retail release, I'm guessing it will be possible. Skyrim was only available for pre-loading a few days before the official release date, so it will probably be the same for Dishonored. I will be surprised if it's possible to preload the game a week before the release date.

Also, depending on your internet connection, it might be faster to download the game the minute it's officially released rather than to preload it. Simply because it can take a while for Steam to decrypt the preloaded data.

Pre-loading is only available a short while before the game is released. You download it and when the hour hits midnight you get to play it.

I don't know what they are planning to do for this. Have to ask a Bethesda community manager or check out their blog which has a lot of information.