I had a problem with setting cell widths in a BIRT 4.2.1 Grid (using Eclipse Juno). I had a grid with two rows and five columns. I had merged all the cells in the top row and found that after that point, setting cell widths on the bottom row made no difference whatsoever - all the cells remained the same width.
The solution was to delete the top row, leaving me with a grid of just one row and five columns. Fix up the cell widths and then add another row at the top and merge those cells.