D&D 3E/3.5 (3.5) Improved trip, two weapons

[Jimlock]

"As if you hadn't used your attack for the trip attempt".

i.e. as if the attack hadn't happened.

You clearly have your own personal way of reading the rules don't you?

SRD:
If you trip an opponent in melee combat, you immediately get a melee attack against that opponent as if you hadn’t used your attack for the trip attempt.


If you trip an opponent in melee combat = If you have managed to trip an opponent in melee combat = If you have managed to trip an opponent in melee combat with a weapon or unarmed.

In your case with a sickle in your off hand...

you immediately get a melee attack against that opponent as if you hadn’t used your attack for the trip attempt.

The "as if" does not negate the fact that you attacked/triped/used-your-off-hand.
The "as if" refers to the free attack attack you gained for managing to trip the opponent so as to explain that you use the same BAB.

Your: "i.e. as if the attack hadn't happened." is false because the attack HAS happened, it was simply used so as to trip the opponent instead of dealing damage to him.

SRD:
Trip
Making a Trip Attack
Make an unarmed melee touch attack against your target. This provokes an attack of opportunity from your target as normal for unarmed attacks.

Tripping with a Weapon
Some weapons can be used to make trip attacks. In this case, you make a melee touch attack with the weapon instead of an unarmed melee touch attack, and you don’t provoke an attack of opportunity.

.......

Therefore, in your example, you USE your off-hand (sickle) to trip, and you USE your primary hand (longsword) to deal damage.

so by the fact that:

PART 1:
When using a weapon in each hand, you usually can't use a shield, which hurts your Armor Class. In addition, you take an attack penalty on attacks you make with your primary hand and (generally) a bigger attack penalty for your off hand. The exact penalties depend on what feats you have and which two weapons you're using; see page 160 in the Player's Handbook. Parts Two and Three also examine two-weapon fighting in detail.

.....you take TWF penalties for using both your off hand (sickle) and your primary hand (longsword)....

 

[Flarlarlar]

Tripping notwithstanding, what happens when you only want to attack with the off hand? Simillar scenario:

Rapier in main hand, sickle in off hand. You do a full attack with just the rapier, so there are no twf penalties here. Then an enemy causes an attack of oportunity from you. Can you attack with the sickle? The "designed" main hand can be changed between rounds, but on this round it's still the one holding the rapier. If you do have the chance to use the sickle, would any penalties apply? I read something about a -4 but I don't know where that came from

 

[Theo R Cwithin]

I don't think there's a definitive rule on that, but I believe the official consensus is that you can freely declare/switch which is your primary weapon and which is your off-hand weapon-- so yes you can AoO with the sickle with no penalties on the attack roll.

This was discussed here about a year ago; in this post I found a few links to RotG and FAQ talking about this (that's where the "consensus" comes from). That thread in general contains a lot of good discussion on the topic of "holding two weapons but not fighting with two weapons", including houserules and the like.