Going out on a limb here....
First, what window manager do you plan on developing for??
Kde and Gnome should be available for AIX. If they are, then you should have access to all the GUI development tools built into KDE and Gnome.
Also, IBM should have a tool for developing for whatever window manager they provide. I think they used to provide motif??
Somoneone will correct me if I'm wrong.
I think all *NIX questions are allowed here..