Yep, what Bob said. And I like Nikko's second thought as well.
I don't think you *need* to state that you digitally modified the print. It's fine to do so, but what you are doing is totally within the realm of "an acceptable photographic practice." On the other hand, if you airbrushed in Elvis cleaning up the graffiti, you'd be creating a photo-illustration vs a photo, and should call it thus.
It's not so much about "ethics" IMHO, but about artistic integrity: the impression you give people about what you made. For example, a friend has two photos, which both look like improbable setups (like they were staged). But one shot actually happened, while the other photo was a collage she made in Photoshop. She's not a photojournalist, and sells these photos as art. But she labels the first a photo and the second a photoillustration. She does this not out of a sense of ethics, but because she wants to clarify that there was a different artistic act performed ("discovering" this funny shot, or imaging it and staging it herself) -- neither is more valid, just different.