From a news article on FoxNews.com:

"In a written statement, the White House said Obama reiterated his concern over the law and how it could lead to a patchwork of different state immigration regulations that would interfere with the federal government's response."

I couldn't agree more: if too many states decide to take responsibility and pass their own laws and use their own resources to enforce them, it could seriously jeopardize the federal government's typical response of attempting to ignore the problem!