Big special thanks to GameFAQs user dsxgate for these images and updates. From what dsxgate says, in order to obtain/use Bulma as a playable character in “Dragon Ball Z 3” (the recently-released Japanese version of “Budokai 3”) you must:

• Complete the entire game; a full 100%… this includes the twenty “Story Reenactments”
• Level-up all DU characters to 99 (?; this is slightly unclear)
• A final capsule will become available (for Bulma)
• Bulma is a third alternate outfit for Videl; she has only two attacks and a Dragon Rush (no ultimate)… but she can fly and charge ki

No, that is not Hiromi Tsuru you hear; since it is an alternate outfit for Videl, it is still Yūko Minaguchi (the voice of Videl).

There are still two months of promotional passwords to come, so hopefully we will be finding out all sorts of other things hidden away in the game.