I think they should be, as presumably what will follow a Yes vote is not a quick exit but some protracted negotiations, and denying the Scots a voice in this in the UK Parliament while they are still part of the country seems very unfair. On the other hand I'd expect the resulting 2015-elected Parliament to focus primarily on Scotland's withdrawal from the UK, and then a second election called as soon as that process is over.
This last might be difficult with the fixed-term Parliaments law, although it's possible -- and indeed surely necessary -- for there to be an exemption. But at any rate, as long as Scotland is officially part of the UK its voters should have a voice in the UK Parliament. Although it might be unlikely, you could imagine a situation where the negotiations are ongoing, but Scotland is still subject to British law and so Parliament might rush through legislation that is certainly not in Scotland's favour. Unlikely, I'm sure, but similar things happen in the bizarre two- or three-month period between the US Presidential Elections and the new President actually taking office where his (sometimes beaten, sometimes unable to stand) predecessor takes a sneaky opportunity to sign off all sorts of laws that the new president would never have passed and which are often against his intended policies.
So yes, come what may this September, I think the Scottish people (and, let's not forget, the English people currently living in Scotland) should have a vote in 2015's election. But if it's a yes vote, then the 2015 election shouldn't be for a five-year fixed term.