One of the main criticisms of using a cap and trade system to fight global warming is that allowing markets in carbon permits and (especially!) in carbon offsets, opens the door to financial manipulations such as we just saw with the real estate derivatives markets. We probably won't get a full blown bubble in carbon offsets, but we will get a lot of wasted money and energy channelled into useless derivatives when we need that money and energy for building a renewable energy economy.
So let's start financial reform with the cap and trade bill. Keep the auctions and aftermarkets as simple and regulated as possible. And in a best case scenario there are no carbon offsets.