If you want to script it, ImageMagick or GraphicsMagick are probably a better bet than trying to hack up something with GIMP batch processing: http://www.imagemagick.com/www/formats.html

If you just want to do basic conversion ignoring layers, it's quite easy:

   for f in *.psd; do
     convert "$f" "${f%%.psd}.png"
   done

You can use find/xargs for this. It'll be faster because you can parallelize it with the -P option, for example, for a 4-core machine:

    find . -iname "*.psd" -print0 | xargs -0 -P 4 -n 1 -I I convert I I.png

For best results, change the number "4" in the above command to the number of cores you have.

The same with GNU Parallel, only it picks the right number of cores automatically and removes .psd from the file name:

  find . -iname "*.psd" -print0 | parallel -0 convert '{}' '{.}.png'

Oh excellent, I didn't know ImageMagick did PSD.

Gimp's support for PSD is understandably poor for the more advanced/modern features, so I wouldn't rely on it without being able to compare the output to something else. I use Apple's Preview utility to manually compare results.