Nosemonkey and Jon Worth have been writing recently about the idea of a UK referendum on its EU membership. Nosemonkey focused on debunking the reasons for a referendum, but there is essentially two reasons put forward against an in/out referendum: it wouldn't settle the membership question, and without a clear "out" plan, such a referendum wouldn't offer any clear choice. I thought I'd add my two eurocents on this, and why I think it doesn't make sense for those supporting EU membership to support an in/out referendum in the UK at the moment.
If there's one thing that can be drawn from the campaigns on the Constitution for Europe and Lisbon Treaty, it's that referendums are very poor ways of deciding the structure of complex, multi-issue matters. Amending Treaties on the EU have always covered very general reforms, mostly on procedure, and it's hard to explain or campaign on such questions. And as Jon noted, being able to fall back on the status quo meant that rejecting Treaty changes was a responsibility-free pass to further your political stance, without the overall answer of the electorate being obvious. It strikes me that if the outcome of a referendum doesn't give you an idea of what people want, then it's not much use.
In the future, changes to the EU Treaties should be on a case-by-case basis, and not general reform Treaties. These would have the advantage of having clear aims that can be debated, and people would have a better understanding of the role the EU plays, and discuss whether it should play a bigger/different/smaller role in that area. This doesn't escape all the problems of uncertain outcomes, but it does greatly reduce them.
In/Out referendums are like general treaty reforms, even if it asks a clear underlying question. This is because the different options to membership - nicely summed up by Jon as the Norwegian, Swiss and US options - are complex results. Nosemonkey has written about Norway and Switerland, and some of the issues with their position. I'll try and write on their relationships with the EU soon (because they have different and complex relationships), but essentially if you're building a single market, it requires common rules, which in turn require common legislation, which in turn requires a common decision-making process.
Switching from EU to EEA membership means joining the internal market but not being part of the institutions that shape it, which is a loss of political power and autonomy.
As Jon points out, the withdrawalist side has to have a case for the alternative which can be debated - without this alternative, the referendum becomes a farce of decision-making. "Do you want the UK to leave the EU?" would translate into "Do you hate the EU?", since it would essentially ask people to state their opinion, rather than actually make a choice.
It would also be unfair to the withdrawalist side. The reason why some supporters of EU membership also support an in/out referendum is that it might be winnable and put the question of membership to rest. Given the non-choice described above, and the general indifference of people to the EU, a yes-leaning "meh" might be the outcome. This wouldn't let the withdrawalist side put their case forward properly, and it wouldn't result in an outcome that would be very meaningful.
Since membership is the status quo, it is essentially for withdrawalist to come up with a case for a non-Member State UK that they want to put to referendum. Until the withdrawalist side come up with an alternative to put to the vote, holding a referendum is pointless, and supporting a referendum without a clear question is merely a more principled-looking way of political point-scoring, whatever side of the debate you're on.