Lispix does PCA on small stacks of images, giving principal component images, and the associated eigenvalues. I've used stacks of 20 images. Lispix should work up to about 100 images.
As of 9/1/09, PCA does not work with file based cubes, but only with cube image windows - the older, memory-based cubes stored image by image. Lispix does not do PCA on spectra, only on images.