While batch processing was already possible using the rather complex Script-Fu, there's now a plug-in for GIMP allowing simple batch processing through a dialog.

DBP David's Batch Processor allows for automatic operations on a collection of image files. Operations such as colour correcting, resizing, cropping, sharpening, and even renaming can be performed with ease, saving them to a different location in a different image format. All operations (except loading and saving) are optional so you can use this for simple image format conversion as well.

DBP can be found here: http://members.ozemail.com.au/~hodsond/dbp.html.
It is mainly designed for linux version of The GIMP but pre-compiled Windows-ports are also available on that page.