Well, maybe he's passed through in his planar travels, but he's never had any remotely extended stay there, and the real point of it all is he knows very little of Sigil beyond the basics.
As for a long background, it's... longer than I thought it would be... :shock: I have it coming, not done yet, and won't finish tonight. Expect it tommorow.
Why not? Sigil has more portals than any Outer plane (which, mind you, are infinitely large), with a few possible exceptions. If you hop through any random portal, there's a decent chance it will lead to Sigil. Basically, if you have traveled the Outer Planes for a while, and you've never been to Sigil, chances are it's because you chose not to come to Sigil. Also, yes this campaign will start in Sigil, although you may not stay there for very long.