An update from the GIMP FAQ:
"When can we see 16-bit per channel support (or better)?
For some industries, especially photography, 24-bit colour depths (8 bits per channel) are a real barrier to entry. Once again, it's GEGL to the rescue. Work on integrating GEGL into GIMP began after 2.4 was released, and will span across several stable releases. This work will be completed in GIMP 3.0, which will have full support for high bit depths. If you need such support now and can't wait, cinepaint and krita support 16 bits per channel now.
It should be noted that for publishing to the web, the current GIMP release is good enough."
Gimp 3.0 will likely appear late in 2011.
GEGL - Wikipedia, the free encyclopedia
Jack