That's interesting: there's no GM product on your list.  I seem to recall that after the demise of the traiditonal NYC Checker cab in the 1970's, the fleet was largely Chrevrolet Caprices or Ford.Mecury Town Car of similar bulk.  Size is a definite factor for the taxi fleet,and it's hard to imagine the prius being large enough to pack people in.
Victoria, BC taxi company is into the Prius in a big way (relative to size of fleet):

http://www.empresstaxi.com/

<snip>...90 passenger vehicles in total, that include over 33 Toyota Prius (Hybrid) vehicles...</snip>