The battery in the kindle 3 does indeed have SDA SCL pins
I just ripped open a kindle 2 to check and it also has 4 pins marked
(+) (-) (D) (C)
So it's a safe bet that the DX is also a smart battery
http://sbworkshop.com/sbdat110.pdf
Honestly I can't give you a complete and total answer but I think err on the side of caution is in order.
Hope that helps.
(I'll try to take some pics)
EDIT: Hmm only have a phone camera. but as you can see 4 pins on both.