While the Ubuntu PowerPC support has already been rocky for years and hasn't been an official architecture since the original Ubuntu Linux days, the desktop PPC images will likely be dropped. The reason for this change is that the Ubuntu desktop image for PowerPC hasn't been building for nearly one month with the latest Ubuntu 13.10 code.
The PowerPC build issues have come down to Canonical's own code with the Unity stack. No one seems to care nor more importantly stepped up to fix it. As a result, Canonical's Steve Langasek is looking at removing PowerPC for the Ubuntu desktop entirely.
Beyond the modern Ubuntu desktop stack not building on PowerPC due to Unity, Langasek says that PowerPC for the Ubuntu desktop is already next to useless. This comes down to the Unity desktop requiring OpenGL hardware acceleration for Compiz, but with PowerPC GL drivers being rare, it's mostly been an affair to run on the Gallium3D LLVMpipe driver. The LLVMpipe software driver on PowerPC is slow -- especially older PPC processors -- so the experience is painfully bad.
At this time there will still be PowerPC images for Ubuntu Server, Lubuntu, and other flavors, but the official Ubuntu desktop flavor for PowerPC is likely to be eliminated by the end of the week, per this mailing list post.