Ok, two machines.  One running OS X 10.3.9, the other running Red Hat Linux 7 (yes, it's that old).
I compiled Ruby 1.8.6 from source on both machines with no errors.
I opted to use the shell script method for execution.  Here is the script I'm using on both machines:
#!/bin/bash
export AWS_ACCESS_KEY_ID=xxxxxxxxxxx
export AWS_SECRET_ACCESS_KEY=xxxxxxxxxxx
cd /usr/local/s3sync
ruby s3cmd.rb -v listbuckets
On the OS X Machine, this executes flawlessly.  On the Linux box, 
the exact same script returns:
list all buckets {}
S3 command failed:
list_all_my_buckets 
With result 403 Forbidden
3sync.rb works on OS X and fails similarly on the Linux box.
Any ideas would be most appreciated.