I want to bump this one and say I would like this feature as well. It is one of the few major missing features in machforms.
Here is a screenshot of one from InfoPath. You select what columns (fields) you want and what type of data should be in them, then the person filling in the form can click an "add row/item" button to add another row. Then a minus button to the right of each row to remove a row.
If you could add some basic logic to the rows it would be great too, like in the example above the Extended column automatically calculates its value by multiplying Quantity and Price columns. Same with the Total at the bottom. At this point it isn't necessary, just getting the repeating table would be a huge step forward.