you could remove the rim strip & see if there is the metal burrs zach is talking about. It should wear on your strip too, if that were the case you would visibly see wear on the strip close to where the eyelet roughness/sharpness is located. I've done the electrical tape trick as well, it works. I prefer the Velox cloth rim strip, but tape might be a few bux cheaper. Cloth tape usually runs about $5. I've never run both, but it sounds like a solid way to eliminate the possibility.
Pretty much everything has been mentioned now to look for... be sure to run your fingers over the entire width of the inside of the tire and look for small slices/wear that go all the way through the tire itself . I've had to turn the tire inside out to put it under stress to find holes or embedded objects. The tire doesn't want to be in this shape, so it stretches out the curvature to let you see at things a bit better.
One last thing that I am sure would've bubbled if you put the tube under water... sheared stems from not having the valve stem sticking out nice & straight. If the stem is at a slight angle, it could get a slice from the rim itself. The stem should be perpendicular to the surface of the rim where it penetrates through. But, again... this kind of issue is fairly obvious when trying to figure out what happened to the the tube.
