In Europe, without influence.
Implausible as it might seem, David Cameron's involvement has been an even bigger catastrophe. Here was a wonderful opportunity for a British prime minister to lead those other countries deeply uncertain about the Merkozy plans, giving them a voice at the table. If there's going to be a "two-speed" Europe, made up primarily of those outside the Eurozone, then Britain ought to be the one that speaks up for them. Cameron instead did exactly what his Eurosceptic bankbenchers wanted him to do: he talked big. And what did he get in return? Absolutely nothing. Rather than winning over the likes of the Swedes, Czechs and Hungarians, all of whom will now be consulting their parliaments over the treaty changes, he turned them off by continuing to insist on the sanctity of the City over everything else. By using his veto he hasn't stopped the Tobin tax, as the French and Germans refused outright to reconsider their position on introducing it. Nor has he stopped the Eurozone members from going through with the changes, which was never his intention anyway. All it adds up to is he doesn't have to try and get an overall treaty change through the Commons, something he was unlikely to manage.
This is a very short-term victory, it's true. In the longer term it leaves Cameron in an unenviable position. Already the Eurosceptics in the party are agitating for more, and who could possibly blame them? By leaving Britain possibly in a gang of one, should the other three countries all sign up to the new pact however unlikely it may seem now, all our influence on future changes has disappeared in a flash. What then, beyond the advantages of the single market, remains the point of staying in the EU? The Conservatives already loathe the social chapter, the working time directive, the common fisheries policy and all the other "regulatory" burdens they imagine are holding British business back, so why stay in when the negatives in their view so overwhelmingly outweigh the positives? The slightly more pragmatic, as Bagehot notes, want a re-negotiation to remove these impediments, while the hardline Europhobes seem to imagine us as Switzerland with nuclear weapons, or Norway without the oil. Both positions especially now seem to be utter fantasy. Having just so thoroughly pissed off the French and annoyed everyone else with his high-handedness, any such attempts at changing our relationship would be the equivalent of pulling teeth; ours.
All of which is to completely ignore the Tories' real paymasters, British business. They too might loathe the regulations the EU sets down, but the vast majority see no point whatsoever in a renegotiation doomed to failure, let alone the myriad of problems that would arise as a result of us pulling out altogether. They'll be looking at what's gone on today, both the apparent suicidal tendencies of the main Eurozone countries as well as the grandstanding of Cameron and be in despair. The CBI has notably so far said "wait and see", which is hardly an endorsement of either position. Should they look back in a few months/years time and see that Cameron has essentially led the country down the path of isolation, it would certainly be enough for them to think about trusting Labour again.
William Hague's old slogan while leader was that his party's position was to be "in Europe, not run by Europe". Cameron's achievement is to be in Europe while having no influence over it. Romano Prodi has described it as "gained freedom, but lost power". Even more apposite is Nosemonkey's observation that at least Chamberlain came back with a piece of paper. It was worthless, but it was something. Cameron has brought back nothing but more problems for himself.