Since some days ago, single-window mode for GIMP 2.8 is feature complete. That means that I'm not planning to add more single-window mode related features into GIMP 2.8 such as those Peter Sikking originally envisioned that I haven't implemented yet. This is how feature development should be done anyway: incremental and continuous improvements. If you go for the whole thing at once, it will take forever to finish what you started. To my knowledge, there are only a few annoying but harmless bugs left to fix before single-window mode is ready to be included in a stable release.
As for GIMP 2.8 development as a whole, we're still on track for a late 2011 or early 2012 release of GIMP 2.8. There are still some things we really need to fix before we can make a release without embarrassing ourselves, like supporting layer masks on layer groups and making it possible to easily move individual layers in a layer group with the move tool.
If you're curious about what single-window mode will be like in GIMP 2.8, I suggest you keep your eyes on gimp.org and look for the GIMP 2.7.3 release which should happen any day now. I also encourage you to file good bug reports if you find any bugs.