Having just watched a snippet of the 2nd Salmond-Darling debate (nothing better to do at this hour) I spotted what may have been key to ensuring the SNP's dominance of Scottish politics for a long time to come. Salmond pointed at Darling and uttered "In bed with the Tory Party! In bed with the Tory Party!" That had to have resonated.
Unless the Scots vote for independence within the next 10 years I think the present spike in support for the SNP will turn out to be an electoral blind alley.
After Labour lose again in 2020 (which seems likely right now) the desire to get the Conservatives out will reassert itself north of the border and in all likelihood Labour will make a strong comeback in time for the 2025 general election.
Ultimately all voting SNP does is make life easier for the Tories as it means fewer Labour MP's in the House of Commons. Eventually that will draw people back to voting Labour but I think it will take most of the next two parliaments for that to happen.