Well, bearing in mind that the polls suggest that Mr Miliband will have a majority of 10 (when you average out the returns and translate that into share of the vote), it seems fairly unlikely that the SNP will be in government.
Particularly when the same polls show that even with their projected measly 8% of the vote, the Lib Dems will get 23-25 MPs, which will still be more than the SNP musters if it trebles its number of MPs. I suspect that the appearance of Jim Murphy as the Labour leader in Scotland will have some positive effect for the party in due course, making a trebling of SNP seat numbers even less likely.
The notion that Labour wouldn't do a deal with the Lib Dems because of their involvement in the current coalition, and would rather team up with the SNP - laying itself open to four years of the press banging on about 65m people in England being the hostages of 5m in Scotland (and similar hyperbolic statements) if it did - is a bit er...., err... odd.
All that would be required would be a swift palace coup against Clegg (if he retains his seat in Sheffield) - doubtless one of the conditions of doing a deal from the Labour side - and the assumption of the leadership by Cable/Hughes/Bruce in his stead. This wouldn't even require a delay in forming the government, since the obvious leadership candidates would all be cabinet members anyway; they would simply add the title 'Deputy Prime Minister and...' ahead of 'Secretary of State for...' in their job description.
And it might not even come to that - you can see a 'Nick Clegg struggled valiantly to restrain the Tories, and this is why I am delighted that he has agreed to be deputy Prime Minister' line emerging. Forging coalitions leads to some very strange things happening - but as things stand, the Lib Dems remain more likely coalition members than the SNP, I'd contend.
That - as another thread on here might suggest - would open a different set of questions about Trident and its replacement...
Sturgeon knows that the polls are likely to close as we near the election, and talk of being a party of national government does little harm - but the wider implication of having policy dictated by the SNP while the English media fumes will make a Labour-SNP coalition very much a second choice option...
(All the above, of course, presumes that a coalition is required and that the polling results translate into the currently projected number of Lib Dem seats).