A buddy of mine has a pickup with a 350. It runs perfect cold. When it gets hot it develops a little hesitation/miss.

Someone suggested the distributor might not be advancing enough, but I'm not sure about that - It doesn't feel like the overall sluggish retarded timing kind of thing to me... Feels more like it's missing on one cylinder.

I thought I'd start by checking the plugs and wires. But also figured I'd throw this out here and get some ideas from you guys about what else to check out after that. What else could cause this - only when hot?

Sounds like your heading in the right direction, assume you are running a carb so not likely any F/I issues. I would make sure the coil output is solid as things heat up and that the wires and boots are insulating properly.
John

HEI or points? if points check dwell.
fresh tune-up? plugs, points, wires, filters...?

if a cylinder is missing after all of that then i'd do a compression check.

oh yea', and check stupid stuff like a burnt wire grounding on a header... BTDT. =)