Ahh, here we go:
https://bugzilla.redhat.com/show_bug.cgi?id=371341 “upgrade to 5.1 breaks autofs for automounted home directories” – see especially comment #13 which describes the symptom, and #25 which explains what the fix is hidden in the records as. Fix appears to be to use kernel kernel-2.6.18-53.1.4 or higher.