The OP was spot on: this 'policy announcement' has, I suspect, less to do with any policy the Tories would (want to) introduce, and is more of a 'dog-whistle' gesture. That's not to say they wouldn't follow through, just that I think they would do so only because they said they would, and not because it is important to them. Simply put, it points out, to those few who may have forgotten with all the hullabaloo around the election, that Brown 'wasn't elected Prime Minister'.
It's been pointed out in the thread already that on constitutional grounds the suggestion is based on a nonsense, but one thing - the (also constitutionally dubious) 'Prime Ministerial Debates' - has meant that Cameron can't be called out on it. Since the debates, which all three leaders signed up to, already give the false impression that we in any way elect a Prime Minister, all three parties are complicit in fostering that notion, and so none of the three parties can point out the strangeness of the suggestion from Cameron.
It's also already been pointed out that the proposal is an attempt to portray the Tories as the 'party of change' - although from my perspective it was misjudged. It seems opportunistic, as well as seeming to be policy created on the basis of the Gordon Brown experience rather than any deeper principle and, ultimately, seems rather timid - as does much of the Conservatives' 'change agenda'. The final point is thrown in to sharp relief by the meteoric rise of Nick Clegg (contrary to popular opinion, the surge appears to have been underway before the first debate). Not that there's altogether all that much real change in their agenda, but the some of the changes they are proposing (particularly constitutionally) have a more genuine air to them - that those changes would be change, and not just a rhetoric of change.
Any predictions for the result?