What to make of this hasty measure? I suppose it could feasibly be seen as a scare tactic on the part of No.10: strengthening the current three line whip by making it very clear that Dave Is Watching You. But it's far more likely that Cameron is appearing in order to set out the sorts of concessions that James mentioned earlier. There are, after all, almost 50 Tory backbenchers signed up to the original motion — 50 potential rebels. The Tory leadership will want to avoid provoking such a significant portion of the parliamentary party.
So expect Cameron to use the intervening days to craft a position on Europe that will sway his backbenchers; a position that speaks of repatriating powers, and perhaps of a potential referendum in the next parliament, given certain conditions. As for other possible areas of concession, Zac Goldsmith has just tweeted that "This will inevitably be a free vote by the time the bell rings". In the circumstances, Mr Goldsmith's breezy confidence may not be misplaced.