Currently multi-site support isn't planned.
If you want simple multi-site-support you could use the localization feature with a custom locale. But with Craft's auto-update's and easy deployment, why would you want to use MSM anyway?
...we're pretty against adding it to Craft... at least in the way EE did
it, where everything is stored on a per-site basis.
However you should take a look at Craft's localization features. With
that, all content can be stored on a per-locale basis, all users can
have per-locale permissions, each locale can have its own base URL,
templates, routes, etc.. Really is pretty powerful. The only thing
missing is the ability to define custom locales, in the event that
your localized sites aren't defined by territory/language, but some
other factor. That's the main thing that's going to be changing ;)
Correct. Initially it did have an MSM-type feature, but we ended up
taking it out, due to it overcomplicating the system.
If you want to run multiple domains/sites off of Craft, you’ll just
need multiple Craft installs.
You can however use one
craft/app/ folder for multiple sites. But since you'd still need multiple databases, configs, plugins, templates... it's probably not worth it, except that you would save a few bytes on files.