A double weapon is a two-handed weapon (assuming appropriate size).
There are two places in the Core Rules where reference is made to treating it as anything other than a two-handed weapon.
One is in the definition of double weapons in the Equipment section:
A character can fight with both ends of a double weapon as if fighting with two weapons, but he or she incurs all the normal attack penalties associated with two-weapon combat, just as though the character were wielding a one-handed weapon and a light weapon. The character can also choose to use a double weapon two handed, attacking with only one end of it.
The other is in the feat text for Power Attack:
(Normally, you treat a double weapon as a one-handed weapon and a light weapon. If you choose to use a double weapon like a two-handed weapon, attacking with only one end of it in a round, you treat it as a two-handed weapon.)
There are two ways we can interpret the Power Attack reference. Either this is a rule about how double weapons are treated, or it is a rule about how double weapons are treated for purposes of the Power Attack feat.
What are some effects of a weapon being a two-handed weapon?
T1. +4 to disarm and sunder checks.
T2. 1.5x Str bonus to damage.
T3. 2-for-1 Power Attack.
What are some effects of a weapon being a light weapon?
L1. -4 to disarm and sunder checks.
L2. No Power Attack.
L3. Reduction in TWF penalties.
L4. Can be Finessed.
Let's say we assume that the text in Power Attack only relates to Power Attack.
This means that a double weapon is treated as a light weapon for the purposes of a/ penalties related to two-weapon fighting, and b/ Power Attack. That's L2 and L3. It means that the double weapon is still treated as a two-handed weapon for anything else - that's T1 and T2.
So if I'm using my quarterstaff as a double weapon, it is treated as a two-handed weapon for purposes of Disarm and Sunder checks - I get a +4 with both the primary and off-hand ends. It is treated as a two-handed weapon for the purpose of Str bonus to damage - I add 1.5x Str bonus to damage with the primary end, and with the off-end, I either add 1.5x for a two-handed weapon, or .5x for an off-hand attack (per the PHB Glossary entry for 'off-hand')... which of the two takes precedence is not stated. I add 1-for-1 Power Attack with the primary end, and 1-for-0 with the off-end. And I reduce my penalties for TWF for having a light off-hand weapon.
Now, if we instead wish to rule that you add 1x Str bonus to damage with the primary end, and .5x with the off-end, the only support for this in the Core Rules is if the double weapon is treated as a one-handed weapon and a light weapon for all purposes, not just for penalties related to TWF and for Power Attack. Which means that the text in Power Attack - Normally, you treat a double weapon as a one-handed weapon and a light weapon. - must be read as globally applicable, not merely to the use of the feat.
Under this reading, I gain no bonus or penalty to disarm and sunder checks with the primary end of my quarterstaff, and a -4 penalty with the off-end. I add 1x Str bonus to damage with the primary end, and .5x with the off-end. I add 1-for-1 Power Attack with the primary end, and 1-for-0 with the off-end. I reduce my penalties for TWF for having a light off-hand weapon. And I can finesse the off-end.
The 3.5 Main FAQ takes this position - normally, you treat a double weapon as a one-handed weapon and a light weapon. Which suggests that they're reading the Power Attack text as globally applicable.
It works for me - I'd rather see the off-end finessed, than 1.5x Str bonus to damage with the primary end.
-Hyp.