There is no guarantee that the Lib Dems will vote down the planned boundary changes in retaliation for the abandonment of House of Lords reform. Contrary to what some claim, no link was made between the policies in the coalition agreement (the AV referendum was the quid pro quo for the boundary changes) and Nick Clegg has previously told MPs that "there can be no justification for maintaining the current inequality between constituencies and voters across the country." This is a powerful position from which to argue that the Lib Dems should not renege on the agreement.
If, however, Clegg vetoes the bill, the final version of which is not due to reach parliament until 2013, who benefits? The most obvious answer is Labour. Without the boundary changes, Miliband's party only needs a lead of three points (on a uniform swing) to win a majority, compared with one of four points under the new constituencies. Conversely, the Tories, who would need a lead of seven points with the changes, would need a lead of 11 points without them.
The reason Labour retain their electoral advantage is that the electoral bias towards the party owes more to differential turnout (fewer people tend to vote in Labour constituencies) and regional factors (the Tory vote is poorly distributed) than it does to unequal constituencies (the coalition plans to fix constituency sizes at around 76,000 voters).
As I've argued before, even if the boundary changes are implemented, the odds are against a Tory majority in 2015. No sitting prime minister has increased their party's share of the vote since 1974, and Cameron is failing to make progress among those groups that refused to support him last time round. But if the boundary changes are abandoned, it is no exaggeration to say that a Tory majority, difficult to achieve at present, becomes impossible. For this reason, it is no surprise that Cameron is determined to push ahead with the bill.