I think I am reading the SRD differently.
The whirlwind attack allows you to make a meelee attack vs. each opponent. However, Trip, Sunder and Disarm all count as possible meelee attacks.
As the feat is written, you would not benefit from Improved Trip while using Whirlwind Attack, since you would not get that extra attack from Improved Trip, since Whirlwind attack does not allow extra attacks from other feats.
But you could still, if surrounded by 8 yahoos, Whirlwind and attack two for damage, trip the next two, disarm the fifth, try and fail to disarm the sixth (and lose your tripping weapon), and finish up with unarmed strikes on numbers 7 and 8.
THe feat says "When you use the full attack action, you can give up your regular attacks and instead make one melee attack at your full base attack bonus against each opponent within reach." To me that means one attack per opponent, and each attack can be different (disarm, trip, sunder or good old-fashioned damage). So you roll the d20 8 times if you are surrounded by 8 people.
That said, I have played with people that interpret that "one meelee attack against each opponent" to mean you roll the d20 once, applying its result to each opponent. If that were the case, a stronger claim could be made for that one attack being the same thing (trip everyone, disarm everyone, sunder the weapons of everyone, or damage everyone). But a) I think that the "roll once" interpretation is incorrect, and b) The "roll many times" interpretation allows for a cool option (a whirlwind of activity, guided by one's intelligence and dexterity) that is worthy of a feat at the end of a 5 feat tree, and not something similar in style to Great Cleave.