How can I distribute objects along a semicircumference in Inkscape?

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

In short:

1. change the center of rotation of the object (the small circle), to set it in the center of the big imaginary circle, around which the objects will be cloned.
2. select your small circle, go to Edit > Clone > Create tiled clones
3. Clone settings: 1 row, 20 columns (or however duplications you need on a full circle)
4. In Shift tab: per row, set shiftX to -100%, per column set shiftY to -100%
5. In Rotation tab: per column: 360 divided by 20 (or your number) = 18 degrees.
6. Click Create

Undo and adapt settings if necessary.