To be honest, I can't see the Chancellor doing it. Leaping from 50p to 70p would be regarded as far too incendiary, not to mention fiscally insane, even for this government. But I can still see them introducing a fair handful of soak-the-rich measures, if only to strengthen their reinvigorated attack line against the Tories.
In which case, I refer you to Polly Toynbee's column from a few months ago, in which she recommended that the 50p rate start at £100,000 instead of £150,000. Now that would also be unwise, fiscally speaking, but it wouldn't be quite so incendiary as a 70p rate. And, as I said at the time, I wouldn't be suprised to see Labour put their name to the measure – or something like it. Watch this space.