Hey guys, I am glad to report that the extended battery for this phone does work with the PDAir sleeve case, with a small catch.

You must NOT use the battery door. This is not really an issue because since it's in a sleeve, there is no chance for the battery or SIM to fall out.
Enjoy the pics, taken with the Dash. I am not a photographer, so the obligatory fuzzy pictures...
Picture #1: The back exposed, with NO cover on.
Picture #2: Side view exposed, no case. Hump phone redux.
Picture #3: Side view, in sleeve case. Notice the hump is barely noticeable!
Enjoy!
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Don't let my wife know I have the following GSM-capable Windows Mobile devices:
Cingular Blackjack, T-Mobile Dash, Symbol MC70, Qtek 9100, T-Mobile SDA.