WP, I recall we had a discussion about this on the Birding Thread a while back, but I'm sure new apps and updates have come out since then. I think both Nat Geo and iBird got favorable reviews. I still haven't uploaded either onto my phone but would like to do so.
I'm also recalling that the bird experts on the thread (Barred_Owl in particular, I think), strongly recommended using headphones for the calls, or at least not playing them out where the birds are, because a bird responding to the recorded call may put itself in danger of predation.
Thanks for including the comment about playback, Biomancer. Unless you're doing some sort of research, playback of bird songs or calls should not be broadcast in the field.
Otherwise, have fun, WP! I only have a dumb phone, so no birding app's for me just yet. Maybe someday...