This is aquamacs 1.5 on a macbook. Exact error when I try to spell-check:
Error: The file "/Library/Application Support/cocoAspell/aspell6-en-6.0-0//en-common.rws" is not in the proper format. Wrong endian order.
ADDED: I indeed had the wrong version of cocoAspell. But installing the right version didn't work until I actually wiped (or renamed) /Library/Application Support/cocoAspell and reinstalled from scratch. That's what had thrown me for a loop. Thanks for the quick answers! Stackoverflow rocks!