It doesn't look like iPad Pages supports merged cells. I couldn't find a way to do it, and when I imported a document with merged cells I got this message:
*Document Import Warnings*
_Pages doesn't support some aspects of the original document._
Merged table cells were unmerged.
Sure enough, the merge was gone.
So it looks like the work around is to turn off text wrapping in a cell and it will flow into empty cells to the right. Of course, if those cells are not empty, they will overwrite the left cell.