That's the latest news as I write, being effectively confirmed by at least three separate and not directly connected sources.
Well, the position he held was embarrassingly bad for him, and he really was a fish out of water as Shadow Chancellor. The best that can come from this is that at least he tried it and found out for himself that it is not "just another job" in national politics. Note that his resignation is said to be "for personal and family reasons" so are apparently nothing to do with his difficulties handling the brief itself.
However, let Johnson's problems with the Shadow Chancellorship be a lesson for all of those who might be tempted to have a go at the real Chancellor of the Exchequer, George Osborne. The position is far more technical than political (and it's very political!); and even the shadow equivalent post is not something that just any old politician can take on easily or successfully!
UPDATE at 5.10 pm: Now confirmed! News on the Shadow Cabinet front is that Ed Balls is to be the replacement Shadow Chancellor (no surprise there!), Yvette Cooper is to go to Home, Wee Dougie Alexander to FCO, Liam "there's no money left" Byrne to Welfare, and Tessa Jowell is to be given extra responsibilites shadowing the cabinet office. This is a very Brownite line-up; and Ed-M will be unable to control Ed-B in any way.
There may be trouble ahead...