Saw this solution yesterday on a different post (from 8/24/09):
"Elizabeth D. from Benicio, CA:
You may think this odd, but a proven solution for many things is WD-40 or even better is Break cleaner spray. If you ever have Sharpie on anything break cleaner will get it off with no scrubbing,then once you clean it, wipe it down again with a non-chemical cleaner to get the solvent smell off the furniture. I even got sharpie off a leather couch with it."
I think she means brake cleaner or brake parts cleaner - something my dad & my husband have in the garage? I'm not sure exactly what it is called but it is a solvent and will get rid of a lot of tough substances.
You might also try hairspray - for ballpoint pen that is my go-to. Works on regular fabric every time, but I'm not sure about leather. Good luck! Goo Gone and Magic Erasers are good suggestions, too.