Can oil/grease damage tires? I've noticed a small puddle of light oil beneath the rear wheel of our Rohloff-equipped tandem. I'm told that a small amount of leakage is normal and I'm not worried about that.

The issue is that a lot of it falls on the rim and the tire. Obviously it's bad news on the rim for braking, so I'll be wiping it off and possibly cleaning with diluted degreaser. 

My question is: is exposure to light oil likely to damage the tire? I know some rubbers can be sensitive to certain chemicals, so is hub oil/bike tires (in this case Schwalbe Marathons) such a combination? I would've thought someone else would have had this problem if so, but I'd like to check.

More generally, are there any products/chemicals should I keep away from tires for fear of degrading the rubber?
Like Daniel said, a good rule of thumb is to keep oils off rubber and plastics. Some will damage them, some will preserve them, but you don't want either type on your tires. You should be more worried about the fact that the oily spot on your rear tire can cause it to break traction much more easily, especially while braking or cornering. If you're going to be storing the bike for any period of time, just stuff a rag between the spokes up next to the hub to catch and absorb the oil.