Here's how I see it:
PHB, p. 84:
"On your action, before making attack rolls for a round, you may choose to subtract a number from all melee attack rolls and add the same number to all melee damage rolls."
Touch spells don't do melee damage. Most of them do some sort of elemental damage. So, I'd say by the core rules, you can't Power Attack a touch spell (PA'd Harm for 5 extra points would be broken)
It's still possible to combine touch attacks and regular ones (example: the psionic power Dissolving Touch combined with monk attacks or the claw powers), and in those cases you should be able to Power Attack the regular attack (which will increase the bludgeoning/slashing damage, not acid)
Note that the psionic Deep Impact/Fell Shot/Unavoidable Strike feats (which let you use weapons as touch attacks for the purposes of AC) are not true touch attacks (and ARE still melee damage), and would still let you use your Power Attack. Good thing, since PA is a prerequisite for Deep Impact.