Is it possible to crossfade songs on an iPhone? Is it possible to crossfade songs on an iPhone that are played through the iPod app?

If there is no native way to do this, are there any apps that can do this?
I'm sorry, I don't think it is possible (yet). If you had an old iPod Nano/Classic then I might have suggested 'Rockbox' (give it a google). Send Apple a feature request about it, they never know, if enough people bombard them with requests then maybe it will become a native feature.

Sorry I can't be of more help.