It's kind of a loophole, and it pre-dates Bush to an extent, but he's turned it into a sort of charade. The executive has always been charged with enforcing the law, where Congress makes the law. It's not as if the executive hasn't "looked the other way" before, especially with regard to enforcement of immigration status.

But Bush's actions have been more numerous and more brazen. By publicly committing to not enforce laws, he's put everything above board. And I'm not sure he can be taken to court over the issue while still in office. He could be impeached, but I doubt Democrats have the votes in Congress for that.

Now that the cat is out of the bag, watch out if there's a Democratic president but a (later) Republican majority in Congress. When President Obama pulls the same game in the other direction, Republicans will reap what they sowed.

It does occur to me that the signing statements are similar to the concept of line item veto. While some say line item veto power would help keep the budget in check, you can also see how one enters a slippery slope of disbalancing the triangle of power.