http://www.katspace.org/graphics/digitart has a number of gimp tutorials, including icon-making tips.

The icon_style comm often has links to (gimp-centric) tutorials in their challenge posts:
http://icon-style.dreamwidth.org/tag/themes
http://icon-style.livejournal.com/tag/themes
Also the winners in the challenges are encouraged to explain how they made their icons; not exactly tutorials, that, but at least they're tips.

Links to GIMP resources and tips are included in this link collection: http://www.katspace.org/links/graphics/