Who is the person who passes in front of the narrator on the automated walkway?

A model for destruction?

It is entirely possible that in the Narrator's lucid state, he was scanning the crowd subconsciously, looking for the image that would become Tyler Durden. Peering upon person to person to find the right image that would become his 'perfect' personality, the basis for his rebirth, the basis of What a man is supposed to look like.

Upon seeing this man, he was infatuated, This is Tyler, he said to himself, subconsciously. And his mind ran with it from there.

If you want to think of it in an even more meta sense, it's entirely possible the Narrator passed Brad Pitt himself and based Tyler off of Brad Pitt's image.

The Sleepwalker

The other, more accepted theory is that, as explained by Tyler towards the end, sometimes he imagines himself watching Tyler, when it is really him committing the action, speaking, etc.

It is possible that the Narrator 'thinks' he is just going his own way to the next monotonous action of his daily job ritual. But his lighter personality is actually 'asleep' and Tyler is awake and ready to start his night of mischief, mayhem, and soap-making.

It is implied that Tyler's soap business is very abundant and been running for a long time when the Narrator first 'meets' Tyler (about a year in the timeline).

It is also implied that the first time the Narrator got some sleep was when he started going to the support groups, a year later, Marla Singer crashes his party and sends him into the downward spiral into Project Mayhem.

To Sum it up

Both theories are very possible, but the more accepted one is the second.

The way it is handled in the book is very different, having the two meet on a beach, never really implying seeing any man as a 'model' for Tyler. This would imply, again, that the second is the actual explanation. Though the first does make for an interesting discussion piece.