Cisco's IOS documentation says that pre-shared keys used for VPNs can be 128 characters long.  If you try to specify a 128 character key this message appears "Pre-shared key length exceeds 127 characters.   Key not added."  So, I have been using 127 character pre-shared keys for a long time.  Then IOS 15 came out and we are still doing VPNs just fine with that version, but not using 127 character pre-shared keys.  It still allows them but the VPN will not come up and "%CRYPTO-4-IKMP_BAD_MESSAGE" is logged, which means the keys do not match.  It now looks like the pre-shared keys cannot be longer than 125 characters.