Today I received the Emerging Power batteries I bought on Amazon. I was glad to see that their rated voltage is 3.7v.
There's no marking anywhere on the battery or packaging for maH rating. Update: I received an email from Emerging Power's sales department saying the nominal rating is 1300 maH. This is about 15% less than the nominal rating of the Epson battery, so I'd guess that the Emerging Power battery won't run the camera for quite as long. Still, it's nice to have spares even if their capacity is a bit less.
Incidentally, the packaging says this same battery is used in a whole flock of other digital cameras including the Fuji 2600, 2800, 4800, 4900, 6800, 6900, MX-1700, -2700, and -2900, and the Leica Digilux Zoom -- so you have some alternative supply sources if you're ever caught out with a powerless R-D1.