This is a straightforward job for an experienced PHP programmer who has done this type of project before and knows what they are doing.
We need a Multiuser/Multisite/Multidomain Plugin programmed for the well known open source script Question2Answer: [url removed, login to view]
The plugin needs the same functionality like the WordPress MU plugin (as integrated in WP 3) + Domain Mapping.
It's easy to create plugins for Question2Answer. Read more: [url removed, login to view]
Some more information about the WP Multisite feature: http://wpdude.com/wordpress-multisite-2