If you're running either NX 3 or NX 4, try this:
Create a rectangular sketch.
Extrude the sketch into a block.
Add blends around the 'top' of the block (if you want to make this bulletproof, use the 'Selection Intent' option of 'Face Edges' when blending the 'top' of the block).
If you're running NX 3, blank the solid and then edit the Sketch. If you're running NX 4, just edit the Sketch 'with rollback'.
While in the sketch editor, add a 3 point arc selecting the end points of one side of the rectangle.
Now delete the side of the rectangle that the arc 'replaced'.
Now leave the sketch task.
If you blanked the solid, unblank it.
Now look at your results. Was this what you expected?
BTW, you can actually do this with NX 2 as well, just that it takes a couple of extra steps, but it does work.
John R. Baker, P.E.
Product 'Evangelist'
NX Product Line
UGS Corp
Cypress, CA