The likelyhood is that if Cameron proposes an in/out EU referendum, Labour will follow suit. The problem for Labour is any referendum is likely to happen on their watch & the usually hostile tabloids make referendums unwinnable for Labour governments.
Improbable as it now seems, both Labour & Tory establishments will make sure there is a tabloid campaign to stay in the EU. Some deal will have to be made with the Murdochs, Desmonds & Dacres. And I am sure that Miliband & Cameron have discussed a deal on this.
Even the Tories know leaving the EU is crazy. Much as they dislike the social creep that protects workers rights and also rightly complain about some of the EU's absurd inefficiencies and democratic deficit, ultimately, the UK needs the influence membership of the EU brings to inter-EU trade & world trade agreements. It also helps our 'special relationship' with the US. As a member we are a useful insider influence for the US. It was this US trojan horse factor that made France in particular, reluctant to admit us in the first place - delaying membership by 12 years. The discovery of strategically important North Sea oil and our desperate willingness to accept poor accession terms finally won them over.
However that does not mean our membership is secure. As Merkel indicates today, the EU is quite willing to kick us out if we demand better terms. We are not in a strong position to make demands.
I can already hear your objectiion that we have a binding treaty with the EU. We cannot be kicked out. Indeed, except the Scotland referendum provides one route. If Scotland has to renegotiate entry as a new country then surely the new country of England, Wales & NI could face a similar call. Scotland would create two new countries by splitting the UK. The new UK is not the same as the old UK. In the same way the Czech republic is not the same as Czechoslovakia. The EU would simply not allow the UK to join. It would only take one country out of the 27 to object.
Alternatively, if Scottish independence fails, as seems likely. The UK might remain nominally part of the EU while increasingly shut out of new trade and social integration. Does this matter? Well, it could be the worst of both worlds, diminished influence while still net contributors to the budget.
Whatever happens, we could be in for a bumpy ride.