You remember right. It is mentioned on PHB p125, Magic Actions, Cast a Spell, Touch Spells in Combat. There is a further commentary on Tome and Blood p79.
To put it briefly touch attacks delivering spells threaten on a 20. If it is a confirmed critical, damage is doubled. If the spell inflicts initial damage and further damage on following rounds, only the initial damage is doubled.
You could improve the threat range by taking Improved Critical in one of the three categories outlined in the FAQ (Ray, Energy missile, Touch Spell). I think the average spell slinger would have better things to burn their feats on by the time they achieve the prequisite BAB for Improved Critical, though.