It is very tricky to fit paragraph text into a circle. Line breaks are never smooth, especially when you have long words. My approach is to use a larger circle, but fit the text inside an imaginary square that just fits inside the circle.
For math geeks, the ratio between the side of the square and the diameter of the circle is the square root of 2. I had to use this proportion recently to fix a web site layout.
Obviously very short headings fit perfectly well inside a circle.
(Yes, yes, circles will come to SlideMagic)