It came naturally to me to use a space separated list so that it is easier to specify it within quotes from command line calls as well as ruby code, for example `mco rpc rpcutil get_facts facts="fact_1 fact_2"` or `mc = rpcclient('rpcutil'); mc.get_facts(:facts => "fact_1 fact2");` 

But if you feel that a comma-separated list will be more intuitive, the change should be trivial and I'm willing to make it.