If a character has Multiform, and is in his base form when granted a UBO power (e.g Flight), then changes to another form, does he maintain use of the UBO power in his new form, or is that only in the form in which he received it?

As a default rule, a character who’s received a Usable By Others power from another character, and then uses Multiform to change his shape/abilities/etc., continues to be able to use the UBO power. However, the GM may change this based on dramatic sense, common sense, game balance sense, 50 Cent, or other considerations.