Images won't open in Adobe external editor
Can someone recommend a fix?
Are you using CS2 or CS3? There might be something weird going on with the emulation if you're using CS2.
Barring that, I'd open up Aperture, set the external to nothing, quit Aperture, reopen, then reset it to PS again. Kind of a pain, but stuff like that has worked in the past for me. Also, do you have the latest version of Aperture (1.5.3)? If not, update.
You might also want to try pointing it to a different external editor – if it works on the alternate one, then it might have something to do with your Photoshop installation, which then might necessitate a reinstall.
My last (well, 2nd last to uninstalling and reinstalling Aperture) suggestion would be to repair permissions and cross my fingers.