The obvious solution is to make yet another list: of people that you have lists of things to talk about with (I've tried such a system - didn't help much). Presumably this would indicate to you that somewhere on your iPod was a list of issues to discuss should you run into the person by chance. Of course, you can make lists, jot notes, and set up reminders all you want; eventually you exhaust the ability of this kind of a system to assist you in every scenario. Ultimately, you have to rely on memory at some level. In this case, obviously, you would have to remember to look at this new list each time you encountered somebody unexpectedly to check and see if he or she was on the list.

It isn't what you want to hear, I'll bet, but it sounds like you've reached the limits of assistance of list-making. It may be that it is time to:

a) rely on your memory

b) be willing to phone or e-mail bob after your chance encounter to talk about that horse should you forget to bring it up in person, or be willing to set up a meeting with bob specifically to discuss the horse.

Finally, your answer to this question may be telling: if the items on the list are important, why can they wait until you happen to run into bob? It seems that, in the spirit of GTD, if you're not willing to make getting in touch with bob an action (rather than passively waiting to encounter him), you shouldn't let your list of equine topics lurk around in the shady areas of your iPod causing you this mental overhead.