Presently, the GIMP is in a
difficult state of growth and transition. The current public release is a highly
stable and useful program that has been available for about a year
now. It is quite extensible for user-programmers , yet a certain
quandary has arisen now that the current developer's releases are going
down an evolutionary path so different that any
plug-ins (extensions) that you might write for the GIMP
will probably be broken when the next public release is issued in a
few weeks 
From what
I've seen on various GIMP mailing lists, the
development team is in a rush to release a new stable public release
to keep up interest in the program, yet there is a tug-of-war going
on as to what buggy features to fix or omit from public
releases, and what new low-level technical issues should be resolved
now so the code won't have to undergo any more radical changes
anytime in the near future.
The current public release, 0.54 is a Motif-based program while all the current development releases rely on GTK or Graphical ToolKit language to avoid forcing people to have to compile the GIMP with Motif. Most of the current set of plug-ins work with the October 1996 developer's release of the GIMP (0.61), yet many of the newer plug-ins are being written to only compile and run with this version. The December 1996 developer's release breaks all plug-ins because the GIMP is being rewritten to use tiled memory, as opposed to the linear format which has been used up till now. This will allow the GIMP to work with files of practically unlimited size ...provided your hard drive still has enough space to swap to when all is said and done.
As a result of this, many people are waiting for a sign to see which way the code is going before writing new plug-ins so they won't have to rewrite them for the next generation of GIMP. The core GIMP is growning by leaps and bounds, but its ``arms and legs'' are stunted and are having trouble keeping up. Well...the program is called the GIMP afterall (that was horrible, I apologize).
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