As of today, the Lisbon Treaty comes into force in Europe and Britain becomes just a province of that supranational authority. Richard North sums it up:
From today, as the Lisbon treaty comes into force, we are no longer masters in our own house. Our prime minister, as a member of the European Council, is obligated under this new treaty to promote the aims and objectives of the European Union, over and above those of the UK, and is bound by the rules of the Union.
Of course, this will make no immediate difference. It simply renders de jure what has been de facto for several decades, but the coming into force of the treaty marks an important symbolic turning point. We are no longer an independent country, de jure. Our prime minister and his government are now working for an alien government, based in Brussels.
+Cranmer has more.