I'll add that I got this response on the GIMP Talk forum:

The gimpfu UI is fairly limited (autogenerated based on the parameter types you supply to register()).
OTOH, if you are using PyGTK to implement your UI, 99% of GTK functionality is available... soooo, everything that you could even think of
So if you use pyGTK, tabs are certainly possible.. Ref here: http://www.pygtk.org/pygtk2tutorial/sec-Notebooks.html

-Rob A>