I went out to eat on Sunday with the family, and somehow someone must have flinged some butter and it landed on my gray reissue, and I was mortified.
I took a chance and did the following, which did remove most of the stain, just be very careful. I used one of those Mr. Clean sponges and got it wet. I wiped it on the stain everytime the leather dried. Don't freak out, it will not leave a waterstain. Once it dried again, I did put some cornstarch on what was left of the stain (by this time most of it was gone).
The butter stain is barely noticeable at this point, and I barely see it unless I examine the bag. I don't think anyone would ever notice the stain.