Looks like a pretty standard Li-poly battery. There's a bunch of outfits that sell similar ones for the iPod and PDA replacement markets, so I'm betting they will show up eventually for the Reader.
Be tough to sell them right now though. The usual usual sales pitch (and what made me replace the battery in my Palm T3) is longer run time
I think the power jack layout may be a "feature." A significant cause of early gadget mortality is walking off with it still connected to the power cord. If you're lucky, it just pops out the plug, if you're not, it snaps the jack off the board internally. If the jack is on the main board, this means big bucks.
The way this is laid out, it would just trash the little sub-board, which would be a very easy and cheap repair.
Or maybe it didn't fit
Cheers,
Bob