Druid takes shape of an Earth Elemental, Casts Produce Flame, and attacks opponent: If the druid succeeds vs Touch AC he inflicts spell damage, but if the druid succeeds against full AC, does he inflict the earth elemental's SLAM damage + spell damage?
When you are holding the charge of a touch spell and next touch something, the spell is discharged.
Technically, produce flame is not a touch spell, but only grants the ability to make touch attacks that deal fire damage:
In addition to providing illumination, the flames can be hurled or used to touch enemies. You can strike an opponent with a melee touch attack, dealing fire damage equal to 1d6 + 1 point per caster level (maximum +5).
The spell does not grant any ability to use the fire damage by any other means, including as a part of natural weapon attack or unarmed strike.
Usually, with a touch spell, you can discharge it with a touch attack or by making a normal (usually unarmed) attack; if the touch attack hits, it only delivers the spell, but if the usual attack hits, it deals its usual damage and delivers the spell. You must choose to make either the touch attack of the usual attack. For a discussion on touch attacks, see the question How do I make a touch attack?
Unless you using house rules, produce flame may not be used in conjuction with the earth elemental's slam attack. This would be a fairly reasonable house rule in terms of fiction, but it might have some strange game balance consequences. Likewise, unless you are using house rules, hitting only the touch AC but missing the normal AC does not have any specific effect when making a usual attack, even one charged with a touch spell. You could houserule this, but with the same warning as above.
First off, the wording of your question suggests that you may have a fundamental misunderstanding of how attacks work. (I could be wrong.) The wording seems to indicate that the druid simply makes an attack roll, compares the result to the enemy's defenses (touch AC vs normal AC) and applies effects accordingly. This is not how attacks work.
Making a touch attack with a spell (or spell effect, like produce flame) and making an attack with a weapon (or natural weapon) are two different actions. Which action you attempt determines the outcome.
With that said, the question is, "Can a druid apply the effects of produce flame to a natural attack while in Wild Shape?" The answer is no. Like Thanuir points out, produce flame isn't a touch spell; it just gives you the ability to make touch attacks and deal damage with them. However, if the druid were to use a touch spell (like contagion), that can be applied through a natural attack.
Alternatively, you may make a normal unarmed attack (or an attack with a natural weapon) while holding a charge. In this case, you aren't considered armed and you provoke attacks of opportunity as normal for the attack. If your unarmed attack or natural weapon attack normally doesn't provoke attacks of opportunity, neither does this attack. If the attack hits, you deal normal damage for your unarmed attack or natural weapon and the spell discharges. If the attack misses, you are still holding the charge.
- Core Rulebook