Matthew Shugart takes on the famously insoluble Midlothian Question. The Economist also has something on this in the current issue (subscribers only).
The dilemma is common to all asymmetrical federalism schemes, including the one Blair has introduced in Britain. Devolution means that the Scottish Parliament (and to a lesser extent the Welsh Assembly -- Northern Ireland is its own story) exercises power over matters that are in the hands of the UK Parliament with respect to England. This raises the question of why Scottish MPs should get to vote on English issues when their constituents will be unaffected. This would become a particularly tricky question if Labour has a majority in the UK as a whole, but not in England, and the Prime Minister of the UK is a Scot (all of which is quite likely in the next Parliament).
Canada has had a version of this problem in relation to the Territories: before self-government gave the Territorial legislatures formal power more or less equal to the provinces, the Federal government had and exercised authority in the North that it could not in the South, but almost all MPs are Southerners. This might be said to be counter-balanced by the disproportionate representation of citizens of the Territories in the House of Commons.
The problem would become more significant if Quebec were given asymmetric constitutional powers. Right now, Quebec operates its own personal income tax and public pension scheme unlike any other province, and has certain other unique administrative powers. But, in theory at least, any other province could exercise the same jurisdictions Quebec does. Asymmetric federalism would create the same problem here that Britain is dealing with.
Matthew points out the problems in a solution that would deprive Scottish MPs of the right to vote on "English" issues. The difficulty is that none of the other solutions work very well either. If England turned strongly against Labour, which is possible, this could cause major problems for the UK.