It wasn't a slipup. I called the Foreign Office who confirmed: it is yet undecided as to whether the G20 will be a finance ministers summit or a head of government one. The invites go out next week, and they may find out the week after. Maybe later. To treble check, I contacted the US Embassy in London - is the president coming to the G20? "As yet his attendance is unconfirmed" was the official reply.
As I say in tomorrow's magazine, it would be devastating for Brown if the word's finance geeks descend on London with their calculators rather than the Messiah. It would deny him of that photo op from heaven. My hunch is that Obama probably will turn up - but as yet, he still hasn't said he'll come. And if he doesn't, I doubt other heads of state will bother either.
P.S. The Foreign Office get in touch to say that the G20 is indeed a Head of Government summit, and that their plan is to have a finance ministers meeting separately. So is Obama definitely coming? That, they say, is "a matter for the US".