From the srd:
Touch Spells and Holding the Charge: If the character doesn't discharge a touch spell on the round the character casts the spell, the character can hold the discharge of the spell (hold the charge) indefinitely.
The character can make touch attacks round after round. The character can touch one friend (or the character can touch his or her self) as a standard action or up to six friends as a full-round action. If the character touches anything with the character's hand while holding a charge, the spell discharges. If the character casts another spell, the touch spell dissipates.
Discharge: A few spells last for a set duration or until triggered or discharged.
(D): If the Duration line ends with "(D)," the character can dismiss the spell at will. The character must be within range of the spell's effect and must speak words of dismissal, which are usually a modified form of the spell's verbal component. If the spell has no verbal component, the character dismisses the spell with a gesture.
Dismissing a spell is a standard action that does not provoke attacks of opportunity. A spell that depends on concentration is dismissible by its very nature, and dismissing it does not require an action (since all the character has to do to end the spell is to stop concentrating).
I read this as touch spells last until discharged and are only dismissible at will if they have a (D) in their duration line. Touching anything and casting another spell also discharge the spell as specifically stated above.