...I contacted Seidio a few days ago regarding the extended battery they are working on for the Eris. This is their reply earlier this afternoon:
Thank you for your inquiry at Seidio products. We are more than happy to assist you with your concern. The two Extended Life Battery are coming soon for HTC Droid Eris will be 1500 mAh and 3500 mAh.
Your business and continued satisfaction is of great importance to us here at Seidioonline Customer Service, so feel free to contact us at (832) 204-1118 with any questions or concerns.
Seidioonline Technical Support
Seidio Customer Service
I called them about 20 minutes ago and was told that they didn't have a definite ETA but that the batteries would be coming out sometime in early January! The 3500 mAh battery will add from 3-4mm in thickness and does and comes with a new cover.
I did ask about pricing, but they were not able give out that info just yet. Hopefully in $60-$80 range - which I think is worth it! I will have to buy a new case for it, but that's fine by me too...