Unfortunately, I think you're screwed. The ink has probably soaked into the leather a bit, and will never come completely out. I'm betting you'll have to get fairly heavy-handed to get any results at all.
There's some stuff sold at home improvement stores (Lowes, Home Depot) in the paint department called Goof-Off. It's a solvent that removes ink and other stuff like that. I'd try some of that stuff on a Q-tip on a small area. Let it dry completely to see the final effect before you do any larger, more noticeable places.
Hope you prove me wrong, and it comes right out! Good luck.
PS... you have burned that pen, haven't you?
Hmm... one more thought... the solvent that dissolves the ink is also going to give it a chance to soak into the leather as well. I'd go sparingly on the solvent, and maybe use a dry towel to blot it up as you go? Easy does it.