"The protesters have chained themselves to the building"
"Stay back! Back I tell you! Or I'll press the button and we'll all go up together! Ah-ha. Ah-ha-ha-ha. AAHAHAHAHAHA!"
"Our new fiscal rules will end boom and bust"
Anyone spot the difference? In theory public commitments work because cutting off our freedom to act gives us power over other people's. The bulldozers aren't going to tear down the old warehouse because doing so would clearly kill the idiots who have manacled themselves to the railings and tossed the key in the river. We're not going to take another step towards the evil genius because "he's just crazy enough to do it".
But we are going to break the 40% borrowing rule (or we broke it already and we're going to rewrite it.) Why? Because it turns out the government had the key in its pockets all along. Making public commitments only works if you genuinely leave yourself with no options. Imagine if the rule had been "Should borrowing go over 40%, we will call a no-confidence vote in the Commons." For right or wrong, it's a rule they'd have stuck to. As it is, with the only penalty being some slight tarnishing of Brown's reputation (which is, let's face it, no worse nowadays than getting dirt on a turd) and with the alternative being taxes or spending cuts, out it goes.
But there's another point here, which is that it actually doesn't matter if there's a rule or not. Either borrowing >40% is bad or it's not. (See here for reasons why not.) Whether or not the government has broken its own rule is secondary to the question of whether it's managing the economy as well as possible. Michael Vaughan, for example, may or may not have imposed on himself a rule against getting out early against South Africa. But if he hasn't, it doesn't make things OK.