In order to expand the variety of curved LEGO bricks you could print, I redesigned http://www.thingiverse.com/thing:573359 in OpenSCAD. By using the Customizer, you can vary the radius of curvature, the number studs in the tangential direction, the number of studs in the radial direction and the thickness of the brick (check out the diagram showing these different parameters). If you want to go beyond the parameters available in the Customizer, you may download the OpenSCAD source code and experiment with all the variables.
As a convenience, I have created a separate thing called Curved LEGO Construction Kit http://www.thingiverse.com/thing:603760 that has files for all the basic curved brick shapes. All of these files were created using the same OpenSCAD file included here.
Thanks to Dan Medwin for the enhanced diagram!
Update 12/21/14: New OpenSCAD file uploaded. The most recent tweak improves the grip between bricks
Note: The version of OpenSCAD used by the Customizer is not the latest so check out the comment by the 'polyhedron' command if you want to update the code. to the latest version.
Designed in OpenSCAD, sliced in Simplify3D, printed on a MakerBot Replicator2 with Elmer's Craft Bond glue stick on a glass build plate with PushPlastic filament and the following settings:
Infill = 20%
Layer height = 0.2
No. shells = 4
Feedrate = 40
Travel Feed = 55
Temperature = 220C
No support is needed.