I have written this library while I was working on a scad project and thought it might be usefull for someone else. It uses the build in openscad $fn functionality of spheres and cylinders to smooth out cube corners.

Version 2014-09-20:
Initial release

Version 2014-10-05:
Performance updates

/*  
    Another round corner cube library by towa  
    Version: 2014-10-05  
    
    rcorn_cube(size, r, f, center)  
        size        : size of the object like in cube-function  
        r as scalar : corner radius of a sphere  
        r[r, h]     : corner radius of a sphere and height of an additional, overlayed cylinder  
        r[r, h, t]  : corner radius of a sphere, height of an additional cylinder and  
                      vertical translation of the cylinder  
        f           : number of facets of sphere and cylinder  
        center      : false if the object should not be centered  

    Examples:  
        translate([0, -60, 0])  
        rcorn_cube([50, 40, 30], 10, 8);  
        translate([0, 0, 0])  
        rcorn_cube([50, 40, 30], [8, 4], 24);  
        translate([0, 60, 0])  
        rcorn_cube([50, 40, 30], [10, 5, -5], 64);  
*/  

// include in your project with "use