> I have also seen a dog that had walked in a puddle containing petrol and they had nibbly feet for a few weeks after
Have a look at your regular walks. We have a lot of cattle round here and seepage from the muck heap causes my dogs to have hot paws, as does walking through fresh cow manure (if we follow the herd along the tracks and lanes). The first time we experienced it the vet told me it's quite common as the urine is quite acid.
Here are the other causes we've experienced:
Nettles and thistles - thistles in particular cause a persistant irritation
Mud - some types worse than others but it's irritating as it dries and again can be a bit acid dependant on source
General allergies
I think it may affect puppies as they still have puppy fur and softer skin on their paws and fairly soft naked skin on their tummies - my now 13th month old used to get sore armpits but all these symptoms have improved now he has more adult hair to give him some extra protection. I also slosh off both dogs paws after walks and make sure to wash off or brush out any mud on tummies. Should't really be a problem in summer should it!!