You can create a table normally then ungroup it. That converts it into a collection of rectangles and lines. You can then select all of the rectangles and use the Change Autoshape command (on the drawing toolbar in the PC version; not sure where it is in which Mac PPT version) to convert the rectangles to rounded corners.
You'd then want to delete the lines, most likely.
Having done this, you no longer have a table, of course. It's a collection of rectangles with text in it. Answered Jul, 08 2011