I'm pretty sure that it's cannon.
I'd definately say it is, but it's possible that Skyrim will take place before Infernal City, in which case, obviously, the events of it would have no bearing on the game.
But as to whether Skyrim will be a direct sequel to Oblivion, my guess is no. I certainly wouldn't expect to play as the same protagonist, and a direct sequel usually implies hat what happens followes directly from the previous game, and you'd have a hard time understanding the story if you were not familiar with Oblivion. The Elder Scrolls has avoided this sort of thing in the past and I see no reason why it would start happening now. But it seems pretty obvious that it will be a sequel, indeed, the trailer seems to indicate that the events of Oblivion will somehow lead up to those of Skyrim, considering the mention of the Oblivion Crisis, but I'm guessing this will be similar to how Morrowind sort of led into Oblivion, and not in a everything that happens now is a direct concequence of the last game sort of way. But of course, I could be mistaken, considering that all we know of Skyrim's story so far is what has been hinted at in the story.