If I'm holding a touch spell (say Shocking Grasp) and I drink a potion does the touch spell dissipate? Essentially, does drinking a potion count as casting a spell or having a spell cast on me?
it would count as having a spell cast on you.
the issue is the wording about holding a charge... on one hand it gives specific circumstances for the spell to discharge,
but on the other hand it says ´any touch discharges the spell´, which would include touching a glass vial (potion or not), and even ANY touch at large, e.g. taking a step, being attacked by an enemy, etc. It doesn´t even ´localize´ WHERE the charge is being held. Combat Maneuvers (NOT delivered by Unarmed Strike, which can deliver held touch spells) are also not on the list of specified delivery methods, but would count under the ´broad´ definition of ´any touch discharges´.
I've generally assumed that in order for this rule to be feasibly used it would require that you touch something not considered on your person or the floor, otherwise the spell would go off automatically when your clothing rubbed against you or would just dissipate into the floor every time. This is the way we tend to do it at my table and it seems to be the only way the rule makes sense, but RAW the spell would automatically dissipate unless you were naked and flying in the air...
The main issue I've seen even with this ruling is what happens when you get grappled? It would seem likely that the grappling creature would be in for quite a shock (couldn't resist XP) whenever it wrapped its tentacles about you, which could be an interesting effect. I'd like to see someone give a clarification for this as the rules in the book don't really work...