Google to the rescue; couple of pages with what seems like good advice:http://www.google.com/search?hl=en&q=best+way+to+get+gum+off+leather
Removing gum from leather by Wonder W.
Use a soft pencil eraser to "erase off" the gum from leather. It will need to be worked a little, as the gum will take on some of the rubber from the eraser, and slowly lose its grip from the leather, but the leather is not harmed. It's also good on couches, purses, shoes, etc.