Keep an eye out for Mugen to possibley come out with one. It doesn't look like Seidio will.

Samsung

With current technology, any significant increase in mAh rating will require an oversized battery.

 

What makes you think Seidio won't put out an extended battery option?

As to the width of the phone - I suppose it will depend on how big of a battery you want, among other things. If you want one of the 3500mAh monsters (Captivate comes with at 1500mAh battery)k you'll definitely need a new back - Seidio claims those batteries "only" add 6mm to phones - of course, the Captivate is only 9-10mm to begin with, so...

But it may be possible to achieve a more modest capacity with out requiring a new back. Perhaps 2000-2200mAh (just a shot in the dark there).

 

Do they have any extended batteries listed along with any of their other Samsung compatible accessories?

Mugen does.

 

Neither vendor is actually selling a battery period, so where or not they have a listing up seems moot.

 

I bet the more signups for alert on availability, the more likely seidio will create something:
seidioonline.com

 

Mugen has a history of making extended batteries for Samsung devices, Seidio does not. Their are several models which did not have extended batteries available "when released". That does not mean that Mugen won't make one for the captivate. It took 4-5 months for Mugen to come out with a battery for my Nokia, and Seidio took a few months to make one for the Droid.

"If" one will be made, it will most likely come from Mugen. Its not just the battery which has to be made, but sometimes the back cover as well.