Two thirds want a referendum - Two thirds of all Britons now support UKIP's call for a referendum on the new European Union reform treaty | You know when you've been quangoed! - The cost to the taxpayers in supporting government quangos has risen from £24.1 billion to £167.5 billion under New Labour | 49% back grammar schools - Nearly half of all voters back grammar schools with only 13% wanting a purely comprehensive system | New poll reveals that the vast majority of people in the UK support UKIP's postion - 69% of the British people now want a referendum on our continued relationship with the European Union | A Blank Cheque - Speed camera executive says they make so much money you won't know what to do with it! | EU rules make deportation of released criminals unlikely - 13% of foreign convicts cannot be deported as they hold EU nationality according to UKIP research | 6 million points of light? - Almost one million drivers have been flashed for cash and now have 6 or more points on their licence | Conservatives Say No to Freedom - A large Contingent of Tories vote for the ban on smoking in Pubs and Private Clubs | Jack is officially all right - New figures show that over a quarter of your council tax bill now goes into funding the gold-plated pensions of public sector workers | Sinister ID - 70 Interrogation Centres will be set up by the government for the introduction of ID Cards | M says No to ID - Stella Rimington the ex-head of MI5 has come out against ID cards saying that they would not make anyone safer

Can we just leave?

Yes, we can. This was clarified in an answer to a written question in the House of Lords from Lord Pearson of Rannoch:

EU Membership : Withdrawal. Lord Pearson of Rannoch asked Her Majesty' Government:

Why, given that the treaties governing NATO and the World Trade Organisation specifically provide for the free and unilateral withdrawal of member states from those organisations, no such provision exists in the Treaties governing membership of the European Union?

Baroness Scotland of Asthal (for the government): We see no need for the treaties governing membership of the European Union to include a specific provision on unilateral withdrawal. It remains open to Parliament to repeal the European Communities Act 1972, the logical consequence of which would be to withdraw from the EU. The terms of such a withdrawal would be for the government to negotiate with the other member states.

In other words, we can just leave with a single vote in the commons.