Is there a forum or community for Ubuntu Application developers?
Chat rooms count.
The Development and Programming section of Ubuntu Forums has a subforum named "Ubuntu Application Development".
You could also consider the ubuntu-app-devel
mailing list and the #ubuntu-app-devel
IRC channel on Freenode (can't post IRC links, so: irc://irc.freenode.net/ubuntu-app-devel
).
Lastly, if it is development, you can always ask here, or on Stack Overflow.