Will Droid Bionic with Extended Battery Fit Inside the Otterbox Defender Case?
No.

My friend got an extended battery so I tried my case with his phone -- the extended battery is really large, so it won't work.  Which is a shame, because the Otterbox Defender case is so large that a hypothetical extended battery Otterbox Defender wouldn't be significantly bulkier than the current Otterbox Defender (regular battery).

If Otterbox made a Defender case specifically for the Bionic with the extended battery, I would buy both that case and an extended battery.  I guess that the market for that specific case is just too small.