I'm trying to place small circles equaly distant along a semicircunference. Inkscape provides basic options to distribute horizontally and vertically, but can I do along a path, or in this case (if it's more easy) along a semicircunference?
You can use Pattern on Path extension or the likewise named live path effect.
Another possibility is to use radial clones, for cloning the small objects/circles around a common center, creating a circle of objects, and then deleting half of the circle, and "uncloning" the remaining objects.
It allows you to choose exactly how many clones you want in a full circle of 360 degrees.
Here is a video that shows how to make radial clones: http://www.youtube.com/watch?v=D0I51hXCdvE
Undo and adapt settings if necessary.