The dedication "For Dawn" refers to producer Dawn Steel, who died of a brain tumor on December 20, 1997, four months before this film's release.
When Maggie asks Seth what he does for a living, he says he is a messenger. In Greek, the word for messenger is "angelos" and is where we get the word angel.
The word "angel" is mentioned just two times throughout the movie, during a brief dialogue between Seth and Cassiel when both are walking by the beach to see the sunset. It's the only one time that they claim clearly their condition as angels.
When Meg Ryan is looking in the mirror and Nicolas Cage is not reflected, this was shot twice with motion control: once with Ryan and Cage, once without the actors, and mixed together. Also, if you look closely enough, you can see a few strands of Ryan's hair morphing in and out where Cage's face was erased.