Guys, this seems to be not a GIMP Portable problem, but the GIMP itself.
I'm experiencing this issue on both local and portable versions of GIMP 2.8
http://img33.imageshack.us/img33/6544/gimpnohelp.png
I've copied the help for 2.6 (the latest available for download now)
to the folder App\gimp\share\gimp\2.0\help - so my actions seem to be right.
When I go to the Preferences - Help System - General - User manual, it says "There's a local installation of the user manual.". But it doesn't work.
Please tell me - can you reproduce this error?
Let's hope that the updated help file for 2.8 will fix this...
Another little issue that I've found:
after each launch GIMP Portable leaves a file with a name like:
%TEMP%\nsdCCE3.tmp\registry.dll
and does not delete it.