There are Python and Perl scripts around somewhere (and it's easy to write, a great project if you're thinking about learning how to write GIMP scripts in any language). And the new Align tool can probably center, for those using GIMP 2.3, but the tool is a bit difficult for most people to figure out (fear not, the UI is still being developed).
But for those who want a quick solution:
Center a Layer:Cut, then paste. The pasted layer comes out centered. (Unfortunately this loses text information, so if it's a text layer this isn't an ideal solution.)
Paste a layer on top of a copy of itself:Do a Layer to Imagesize before copying. Then copies of the layer will overlap the original.
[ 19:51 Mar 14, 2006 More gimp | permalink to this entry | ]