"P!" is the logo that appears on Haruko's yellow vespa. "P!" is a logo used by the j-rock band, "The Pillows," also made famous by the anime series.*
person2: "P!, The Pillows."
Was it created by the animators as spontaneous endorsement? Is it an infamously old band logo resurfaced? Originally some confused representation of Piaggio-Vespa? probably the most former, everyone seems to be going with it.
While not popularly recognized in it's own country, "P!" seems to have a runaway success story in the United States due to the promotion of American anime and J-rock fans alike- the result being that FLCL, The Pillows and the "P!" logo seem to carry a synonyms relationship.
Pass it on, because nobody seems to be complaining about it.