I think Harry will receive the flying motorcycle as a birthday gift
from Hagrid.
My reasoning: Hagrid borrowed the bike to take Harry from Godric's
Hollow to Surrey. He told Dumbledore he needed to return to bike to
Sirius. However, doesn't Hagrid later tell someone that Sirius told
him to keep the bike... that he wouldn't need it? And we all know
Hagrid sends Harry particularly useful gifts. Transportation is going
to be a big issue, at least at the begining of "Deathly Hallows", so
this looks like a good solution to me.
What do you think?