Martin is one of the core GIMP developers but this past spring after coming up with this release schedule he ended up becoming too busy with other work to contribute to GIMP on a daily basis, which left this free software project with less than three dedicated developers. That ended up being a significant setback for the GIMP project and has now pushed back the GIMP 2.8 release by at least a month or more.
The notes from Libre Graphics World says the only big chunk of work left to finish up with the 2.8 release is the optional single-window mode interface, but the final design specification for this user-interface isn't even completed yet either.
"Given that vector layers and SIOX (foreground selection tool) improvements from GSoC2009 project are likely to be postponed till 2.10-3.0, release of 2.8 looks like 1.5 months of work of a single person full-time (8 hours a day). This is based on the figures from the last estimation."
So in reality we probably will not see GIMP 2.8 until at least March, but don't be surprised if it takes longer than that to get this major update out the door.
Besides the single-window mode, other GIMP 2.8 features include the text tool working on canvas, Cairo-based tool rendering on canvas, a new cage transform tool, improved brush dynamics, tagging of assets, layer groups, and a re-factored developer API, among other changes and bug-fixes.