Diirk said:"A creature can generally notice the presence of an active invisible creature within 30 feet with a DC 20 Spot check. The observer gains a hunch that “something’s there” but can’t see it or target it accurately with an attack. A creature who is holding still is very hard to notice (DC 30). An inanimate object, an unliving creature holding still, or a completely immobile creature is even harder to spot (DC 40). It’s practically impossible (+20 DC) to pinpoint an invisible creature’s location with a Spot check, and even if a character succeeds on such a check, the invisible creature still benefits from total concealment (50% miss chance)."
So even if you make the spot check, it only lets you know that there's an invisible creature somewhere in the area. But you already knew that... so whats the point? If you want to actually pinpoint what square he's in, its +20 on the spot check (DC 40).
Diirk said:Plus, taking a spot check to see something you've already failed to see is a move action. Then moving around hoping to bump into him is another move action. That leaves you kinda short on attacks.
Seeten said:I'd do it this way: The invisible attacker can choose to allow you into the square, by moving. Like moving through an allied square. the reason you cant move through an enemy square is that they wont let you, generally, not that they are mystically different, so if they let you, it'd be just like an allied or empty square.
Thats how I see it, anyway.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.