As previously stated, I don't purport to know much about the U.S. Constitution (just more than the average high schooler, really), but I will say that regardless of what side of the universal healthcare issue you stand on, you should be able to see that one could interpret parts of the Constitution to authorize the recent bill. I'm not saying you have to agree, just that any reasonable person should be able to see that it is a possible interpretation of the text.
Please note Article I: Congress shall have power to lay tax... to provide for the common defense and general welfare of the United States. Furthermore, we know that the healthcare industry is actively involved in interstate commerce, so any kind of healthcare reform involving regulation of the companies themselves could be seen as explicitly authorized under the commerce clause. Just a thought.
I get it, though. People think that if Congress can now tell people that they have to purchase a certain good or service that Congress is basically claiming limitless power. But I personally don't believe that. The text of the Constitution explicitly reserves certain authority to the Executive, the Judiciary, to the States, and most importantly, to the people. Furthermore, we cannot forget that it is the people who have the power to elect our own representatives. It is not a perfect system. Believe me, I know it is flawed in many ways. But I, for one, am grateful that I have a voice (however small it may be), and that I live in a country where we decide things by a majority vote and where healthcare is something that is available at all. I feel this way because I have been to places where the people have no voice and where people die of stupid diseases because, not only is there no health insurance, there is no medical care at all.
All that to say, I think it is important to remember, as we get all hot and bothered about U.S. politics, that what we have going on here in the U.S.- like or not- is a lot better than what much of the rest of the world is dealing with.