There's a post about that somewhere in these forums. What I remember from it is that right now Cegui uses rectangular regions for everything. In order to have your text fit within your rounded corners you have to shrink the inner portion of the frame. This would be within the .looknfeel file.
Open TaharezLook.looknfeel. The first item is TaharezLook/Button. The first ImagerySection defines the label at absolute positions (0,0) and a width and height of 100%. Then it defines the edges the same way. If your corner are rounded then a portion of the label would stick out from the corners.
I also plan on using rounded corners so will face that same problem. My plan is to create my own looknfeel. The corners will have fixed height width, determined by the image height and width. The "label" part would then have:
- left: 0 + image width
- top: 0 + image height
- width: widget width - image width - image width
- height: widget height - image heigh - image height
Hopefully this makes sense.