I think there's a distinct possibility that Brown could pull through simply because Cameron appears to be completely useless (oh please, his entire campaign thus far has been almost solely soundbites). Fortunately people seem to be seeing through his PR machine.
However, if (or rather when) he becomes Premier, Brown doesn't have to call an election until 2010 (though the general trend of the last 60yrs has nearly always been to call one after 4 years).
A lot can happen in 4 years - individual issues or events can be quite decisive in the political climate (look, for example, to Thatcher's boom in popularity after the Fawklands war, or Wilson's sharp popularity boost [and Heath's sharp decline] after his intervention in the '73 industrial dispute).
If, say, Brown were to call a general election soon after becoming Premier, I think he would stand a chance, but not necessarily hands down. I think he'd probably win - though not by miles.