The message behind 'On Your Mark'

I've read through quite a few reviews and a full interview with Miyazaki about this short-film.

The basic message being portrayed in this movie is to never give up [on] hope. The angel is somewhat of a symbolic representation of hope in a chaotic world.

Also, Miyazaki said there is no romantic message in the movie.

Similar to some of his other works (i.e. Porco Rosso), Miyazaki also portrayed his beliefs regarding religion and the future of humanity (i.e. he doesn't like them very much :P).