There are two things I would say to this.
The more interesting one - for me at least - is the fact Milliband has backed himself into a corner by saying so unequivocally during the debates that he would NOT, under any circumstances, form a coalition with the SNP. I think that is a crass position for him to have taken, personally, but he said it, loud and clear, a number of times. So he will have a decision to make, if he finds himself in that position. That could end up being his equivalent of the LibDem No Tuition Fees promise, the ludicrous promise he will not be able to keep that will undermine his credibility from the very beginning. Or he can have the courage of his convictions and end up losing out on the ability to form a coalition.
I think it is crass because, actually, the SNP and Labour agree on a hell of a lot, pretty much everything except the question of the union, and in fact the SNP are to the left of Labour. I can see why he did it, to scare people into voting Labour, but I dont think it will work.
The other thing is, I personally think the Tories will do better than everyone is expecting, and I think the arithmetic will work for a similar coalition to the one we have now. The LibDems will lose seats, the Tories will gain seats, I think those will happen in more or less equal number. I think Labour are to the left of the British public and all the scaremongering, the bullshit about "the mess Labour left us with" and "we could end up like Greece," will have the intended effect and people will vote for the devil they know. Not what I want to see happen but what I think will happen.
For months I have been dreading a Conservative - UKIP coalition. But it looks like UKIP support has collapsed to some extent - but the Tories are likely to be the biggest beneficiaries of that.