To place PowerPoint shapes in a semi-circle position, insert the first shape onto the slide. Right-click the shape and select "Copy." Right-click the slide and select "Paste" to add a second image ...

PowerPoint 2007 has WordArt tools that let you generate text not only in a half-circle, but in all sorts of rings, waves, slants and fades so you can morph your words into whatever shape you envision.