Ed - this is a difficult one!
Art is something you create - the tools that you use are just a means to an end. Whether you are creating a wood sculpture, painting or producing something digitally - what you create is art.
Art can be to someone something divine and to another a piece of trash - what ever the persons reaction does not make something any more or any less Art.
I think digital art has opened the whole image creating field up and made it more accessable to more and more people.
Filters can be used by two different people with the same image with different results - I think that is what makes the viable as part of 'digital art'.
Its almost like a vision that people are creating. I find the main difficulty is transfereing what is on the screen on to some kind of solid medium.
May be another part of the question is when does it become art - while it is on the screen or when its outputed to paper/canvas?
Sorry if I went off the rails

Clare