I can't find anything explicit about whether a touch melee spell is considered a melee attack or not.
Yes, they are attacks.
By the book, a touch attack uses an attack roll. Attack rolls, on the other side...
So, you use Dex if Ranged, or Str if Melee.
You add this only to the attack roll, not to the damage.
A Note on Game Design
Just a few extra pointers about the game design behind of this:
A few people have argued that Str on the Attack Roll on this case would not make sense. While I agree that this can be sometimes more a thing about precision than brute power, we need to see from where "power" comes from.
In real life, when you throw an attack, the power of the blow is dependent on three things: Where (if) you hit, the weight of the weapon - heavier weapons do more damage - and how fast your attack lands. Momentum is the product of Mass and Speed. D&D and a few other RPG's use Strength as a measurement of attack power, so they implicitly tie the speed of a blow to the character muscle power. You can't do damage by moving a big sword slowly.
Dexterity, on the other hand, is about muscle precision and control. It is not about speed. A (real life) archer do have high dexterity, but if you see an archer firing you will see that the most important thing to make an arrow hit the target is how you control your hands, not how "fast" you do this.
This logic reflects itself on D&D attack rolls. Attacks on which aim is more important than the speed of the blow (like ranged attacks with arrows and spells) normally use Dex, while attacks where the speed of the blow is the most important thing (as to bypass armor) use Str.
Throw-Weapons attacks are a hybrid of this. They use both Dex (to aim, the precision) and Str (to damage, the speed of the throw). Bows and Rays are like firearms - the damage (and the power of projectile) is independent of the muscle power of the one that fires them.
That's why D&D (and it's derivatives) permits that you add your Str to your touch attacks. A low Str Wizard will throw a slow punch, and since will be easily dodged, while a better than average Wizard will be able to make a fast movement and hit his enemy before it can dodge. A Wizard that chooses Weapon Finesse to touch attacks is doing something different - he is using a well timed attack and a calculated move, not raw speed, to land a blow. But doing that is harder, so that's why it needs a feat.
In short melee touch attacks use Str because it is an unarmed melee attack. If it is a ranged touch, it uses Dex for calculating bonus to attack.
This also may help clarify: http://www.enworld.org/forum/showthread.php?257525-Touch-Attacks
D&D works by exception - it states general rules, and then they apply unless an explicit exception is stated. There is no exception stated to touch attacks vis-a-vis normal attacks except in how they are defended against. Therefore a melee touch attack uses your usual melee bonuses and a ranged touch attack uses your usual ranged bonuses (most often including STR and DEX bonuses, but of course there are exceptions). The attack itself is therefore like any normal attack roll, but applying defenses can be complicated.
As a wizard in Pathfinder who frequently uses touch spells, not melee attacks, I worked out with the DM that my Intelligence modifier is best used to calculate my attack when using these spells. I think it is only logical that a caster would use their primary ability to hit with touch attacks since melee and ranged classes use strength and dexterity. While my touch spell to hit is still lower than these other classes, I still manage to keep up damage-wise with what they are doing.
While the rulebook doesn't say anything useful in that respect, common sense decidedly says: "No".
Although the book appears to say "all other modifiers, such as your size modifier, Dexterity modifier, and deflection bonus (if any) apply normally", it really only says that all defensive modifiers apply normally, in particular the dexterity modifier. Which makes perfect sense. It doesn't say anything about attack modifiers, though admittedly it doesn't explicitly say that the strength bonus does not apply either.
Applying a strength modifier to the hit roll however does not make sense (though one might allow a dexterity modifier similar to a ranged attack).
The reason why a melee attack includes a strength modifier in the first place is that by definition hitting is not just making some haphazard contact between your opponent and your weapon, but hitting the opponent hard enough as to puncture/punch through the armour, worn or natural, and do actual damage.
Now, in order to inflict damage, you normally need to act with sufficient force, so hitting someone with greater strength is quite obviously more likely to deal damage. Insofar, a strength bonus makes sense.
For a touch attack, this does not apply. Armour does not count, since you do not need to punch through the armour, you merely need to touch the target -- tapping on the breastplate is enough.