Hardware acceleration for GIMP already 2012
This work by Richard Hirner is licensed under the Attribution-NonCommercial 3.0 Unported terms.
Victor Oliveira, who successfully completed the Google Summer of Code project about parallelization via OpenCL support for GEGL, will continue development due to a partnership with AMD.
According to gimp.org, the plan is to have OpenCL-accelerated projection and compositing operations, some basic filters and a new API for OpenCL-based filters. All new code will be open-source.
Most of the work is intended to be done by March 2012, so we can hope that GIMP will gain a huge performance boost by supporting multi-core CPUs and GPU acceleration really soon.