More of an answer to mowhok1, I'm afraid, than to the question itself.
The Scottish Executive's figures show that, during the fiscal year 2004-05 (the latest for which figures are currently available), total tax receipts EXCLUDING North Sea oil were £36.4bn (8.1% of the UK total). INCLUDING North Sea oil, they were £47.6bn (~9.3% of UK total). This obviously includes revenues from things like the Whisky and tourism industries.
Total government spending attributable to Scotland was £47.7bn (or 9.7% of the UK total).
(These figures include local government receipts/spending.)
So if we include North sea oil, Scotland pays about £100m less in tax than it gets back in public spending. That doesn't sound like much of a difference. However, there are a couple of things to note:
1) The spending figure doesn't include government spending in relation to the North Sea oilfields. Probably not a huge amount in the grand scheme of things, but it all adds up.
2) If we take North Sea oil revenue out, there's a deficit of £11.3bn. Even in the highly unlikely event of a newly-independent Scotland having managing to secure 100% of North sea oil revenue for its own coffers, we will still have to find a way to plug that gap within the next 30 years.
3) Another worrying statistic is that Scotland, with 8.5% of the UK population, accounts for only 8.2% of its GDP. Are some people spending too much time in the office spent writing drivel on Yahoo Answers instead of working?
Of course, one can argue that going it alone will be a massive boost to the economy, and that Scoltand the Brand will flourish free from the shackles of its Sassenach occupiers.
Perhaps. I don't care, anyway. As soon as I get my first novel published and don't have to do any more bloody work ever again I'm getting out of this whole cr@phouse of a country (the UK, that is) to live the life of a nomad/international playboy!