Simon Wren-Lewis digests the Treasury's report on the relationship an independent Scotland would have with sterling and the Bank of England  (rUK is his shorthand for the UK excluding Scotland):
The real problem for Scotland is that, in forming a sterling currency union, it will be dealing with a government that thinks like Germany. What is worse, although Germany can sometimes be persuaded to go against its austerity instincts for the sake of European unity, after an independence vote rUK is unlikely to let its heart strings be pulled in a similar way! The problem for Scotland is that the rUK can provide something that in fact costs it very little, but the absence of which would cost Scotland a great deal, so rUK will be able to ask for a high price. Unless the new Scottish government is prepared to pay for a Bank of England LOLR role with some of its oil revenues, it may find it has nothing to bargain with. If no agreement can be found, the Treasury paper is quite right to conclude that using sterling unilaterally would not be attractive for Scotland. So rather than accept damaging fiscal restrictions, the new Scottish government may end up with its own currency after all.
George Osborne is right to present the Treasury's analysis as a stumbling block for the SNP, in other words; but he's slightly disingenuous in pretending that he has nothing to do with that fact.
As Wren-Lewis points out, politics muddies all waters. Normally, the study of Optimal Currency Areas is relatively simple: the more alike two countries are, the greater the benefits of having a shared currency, and the lesser the disbenefits. That's why the euro is such a startlingly bad idea on paper – there's basically no possible group of countries less economically, socially and politically coherent than the eurozone . And in in nations like rUK and Scotland, which are pretty damn similar, the whole deal comes down to one basic question: is the loss of flexibility of monetary policy a worthwhile cost to pay for those benefits?
But that assumes that there is any way a currency union between rUK and Scotland could actually exist as a union. It's a political, not an economic question: what possible scenarios can we imagine in which the Bank of England would view the Scottish economy as anything other than entirely subordinate to the interests of rUK's? And, moreover, what possible scenarios can we imagine in which an actually existing rUK government – as short-termist, economically-illiterate and vindictive as they tend to be – would allow that to happen?
It may just be the case that the rUK would be better off if the Bank of England carried its policy equitably, allowing the rUK to take a hit to ensure the continued strength of Scotland, which would be on of its biggest, if not the biggest, trading partner. But it's nigh-on impossible to imagine a rUK government letting the Bank of England incorporate that rationalisation into its mandate.
Compared to questions of fair allocation of public debt, North Sea Oil and monarchs (Scotland can have em, frankly), the question of Scottish monetary policy might seem boringly technical. But it's one of the most intractable problems standing in the way of Alex Salmond's dream.