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Try one of those Mr. Clean magic erasers. I don't know what voodoo magic they use, but I've been impressed in the past at the kinds of things they have worked on. Though I've never tried it on a leather seat before.
I guess I never finished the thread, but ultimately I purchased this: Ink & Stain Remover
It did an adequate job even on 3 week old ink that it is not noticable any more and didn't seem to damage the leather. Expensive ($25 with shipping) but sure worked well. I'd try some of the other suggestions if the ink hasn't been there too long.