This is a known bug (SPL-68932) in Splunk 5.0.3. The message is rather harmless, and your _SYSLOG_ROUTING should still works as usual.
You can either ignore the message during splunk startup, or by adding the following entries in your transforms.conf to make the message go away:
[accepted_keys]
is_valid=_SYSLOG_ROUTING
More details on this [accepted_keys] stanza here:
http://docs.splunk.com/Documentation/Splunk/5.0.3/Admin/Transformsconf
Once you have made the above changes and restart splunk, the warning messages should go away.
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