Incredibly, this is actually starting to happen in dozens of major cities all across the United States. It may be difficult to believe, but in many large urban areas today, if you are found guilty of "public camping" you can be taken directly to jail. In some cities, activities such as "blocking a walkway" or creating any sort of "temporary structure for human habitation" are also considered to be serious crimes. And there are some communities that have even made it illegal to feed the homeless without an official permit. Unfortunately, as the U.S. economy continues to slow down the number of homeless people will continue to grow, and so this is a crisis that is only going to grow in size and scope.
Of course the goal of many of these laws is to get the homeless to go somewhere else. But as these laws start to multiply all across the nation, pretty soon there won't be too many places left where it is actually legal to be homeless.