I tend to be of the opinion that in the sort of scenarios discussed above, the ambush rules are overkill. The combatants-to-be are primed and ready, and it seems to me that although it may only take seconds (or even fractions of a second) it takes to draw a weapon, gather will, or swing a punch, it takes far less time to duck or dodge. The ambush rules are fine for total surprise situations where the defender is unprepared or unable to defend themselves (ie, true ambush situations, or possibly even sudden attacks during an otherwise peaceful conversation that gave no reason to anticipate sudden violence).
I think that simply attacking first should give one or both of the following advantages:
First, whoever decides to attack first gets the first swing. For the first exchange, the person who declared first has the initiative automatically (or optionally, perhaps treat this as 'Supreme Initiative' from Supernatural Speed).
Second, if the attacker was trying for a sucker punch, then give him a 'free' maneuver to place an aspect on a target of his choice to reflect the sudden nature of the attack (ex: "Didn't see that one coming, didja?"). This maneuver (which should probably involve the attacker's Deceit against the target's Alertness or Empathy, and ) is assumed to take place during the conversation (or social combat) immediately before the transition to physical combat, and can be invoked for the usual bonus to the attacker's first attack.
If the defender has their guard down, then the normal ambush rules are probably more appropriate. An example of this might be meeting someone (who doesn't recognize you as an enemy) in a bar, chatting amicably for a bit, then suddenly clubbing them over the head with a beer bottle.
At least, those are my thoughts.