Hi,
Ones upon a time we have To Sphere button on the Mesh Tools area to move vertices selected outlining a circle.
Now I don’t see in 2.62 anything like this!
Tx!
Spacebar / search for sphere or just Shift+Alt+S
If you need a button for it on the toolshelf just write a python script. Blendercookie has a number of tutorials showing you how
Grazie! 