Not in current (pre 2007) versions

You have just a bit of minor control over the layouts with additional
content areas.

Basically, those areas will fill the area taken up by the placeholder on the
slide master. So if that placeholder on the slide master is larger, the
content areas when the various "title + content" slide layouts are applied
will be larger also. If that placeholder is smaller, the various content
placeholders in the "title + content" layouts will be smaller, too. But
unfortunately, we have no more control than that; we can't reposition the
content areas or anything.

You might be glad to know that in PPT 2007 (currently in beta), we can
create our own slide layouts, and PPT comes with, mmmm, usually about 9
(depending on the theme or template) to start us out.