While the PowerPC version of Ubuntu isn't officially maintained, but rather is considered a community port, Canonical's kernel team is the one that ends up spinning the PowerPC kernels alongside the other architectures. In order to reduce the kernel team's maintenance burden, they want to do away with the non-SMP PowerPC kernel for the 12.04 release.
At the moment Ubuntu on PowerPC has a non-SMP kernel, SMP kernel, and a PowerPC64 SMP kernel. They are just looking to get rid of the kernel that doesn't handle multi-core computing. Their justification on the maintenance burden is that each PowerPC kernel build takes approximately two hours.
Dropping this kernel shouldn't impact any older PowerPC systems as the SMP kernel should work on single-core PowerPC systems, but there might be a performance penalty due to additional overhead.
As said on the mailing list, "We'd like to drop this as soon as possible, in time for Beta-2 would be ideal."
Another kernel-related change that happened on the x86 side for Ubuntu 12.04 LTS is relating to 32-bit PAE support. At first they wanted to drop the non-PAE i686 kernel from Ubuntu 12.04 LTS since most hardware these days supports Physical Address Extensions, which allows addressing more than 4GB of system memory in a 32-bit environment. However, in the end they decided to postpone the drop until Ubuntu 12.10 and for the 12.04 cycle the 32-bit version will just default to the PAE-enabled kernel build. You can also check out the Phoronix benchmarks of Ubuntu 12.04 on i686, i686 PAE, and x86_64.