I had a case the other night where a player wanted to move one 5 ft square 'diagonally' whilst in melee.
Is this 5 ft (no AoO) or 7 ft as pythagoras or 10 ft (step west + north)? The later two would draw an AoO.
Any opinions, or better still point out a rule on this? (I miss hexes!)

I thought a 5 ft step was to any one of the 8 squares adjacent to the one you're standing in, not just the four cardinal directions...

I don't personally see the 5 foot diagonal step to an adjacent square provoking an attack of oppertunity unless someone is defending the square the character is stepping into. Now if the character (Bob)and his adversary (hobgob) were, say, stading face to face and he (Bob) wanted to move to the side and then back (10 ft step) it would be an attack of oppertunity because the square he (Bob) is stepping to the side into is defended by his adversary (hobgob). All 8 squares surrounding a character are defended, so if someone moves through one to another, it would provoke an AoO. A diagonal move to another square, if you wanted to make it so, wouldn't normally provoke an AoA unless the other square was defended by a different foe (say a troll). However, if you wanted to say that the character (Bob) strayed into both the sqare beside him and the square directly behind him while moving to the diagonal square behind him, then such a ruling would allow for an AoO from his original enemy (hobgob).

Right.... I think that clears it all up then. In my campaings I play it so that all 8 squares around the character don't draw an AoO.

OK, fair enough and that does seem to fit with Eric Noah's version.
However, when you move from small scale to large scale do you apply the same rule?
i.e. assume 5ft squares. 6 squares N, S, E or West is 30ft (typical movement) How about 6 diagonal squares (eg SW)? Isn't that more like 40ft while a 30ft move is 4 squares?

I would say that as long as you apply whatever rule you want consistently it remains fair for all. I've always felt that moving to an adjacent square is considered 5 feet even though trigonometrically that's not accurate for the diagonals.

I would think you would have to go e, n, e, etc. or whatever direction you are going...

In the PHB there is a couple of examples from the magazine 'Dragon'. In the examples showing AoO they have this Tordek doing a "Five-Foot Step" diagonally, so I guess math is thrown out the window :)

And Erran that's funny. I can see it now:
<player> "Ok, guys lets get out this place. Remember to move diagonally so you'll cover more ground"

Out of curiosity, why did the player want to move one "square"? Was this a boardgame type? Seems to me that if it was actual PnP you need not define squares or hexes or any such, simply define in terms of actual movement.

Capstone: In DnD3e combat, where you have things like AoO, flanking etc., it simplifies matters a lot if you use a square grid to place your miniatures or counters on. The reason he wanted to move could for example have been to position himself opposite of another PC, thereby gaining a +2 flanking bonus to attack rolls on the enemy right between them. If he was a rogue he would also have gained the +Xd6 sneak attack bonus then.

Exactly right Azardu, and since we were playing online with every player in a different country the grid system is the best way of unambiguously describing where everyone is.

Erran: Do you use chess-like coordinates or something? I'm a bit curious as to how you manage this in a chatroom session.

Yeah, along the lines of: You have entered a rectangular room from e3-g3 and e7-g7. There is an orc at f4 ....
Seems to work ok. The alternative is Open RPG but that may be more trouble than its worth.