With the 580 in slave mode hold the mode button for a couple of seconds and it will go into manual. It's all in the manual (as it were).
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You will love this:
Dear Mr. Dallam:
Thank you for contacting Canon product support. We value you as a Canon
customer and appreciate the opportunity to assist you with your ST-E2
and 580EX.
You cannot use the 580EX as a manual flash when it is set to Slave.
We hope this information is helpful to you. Please let us know if we
can be of any further assistance with your equipment.
Thank you for choosing Canon.
Sincerely,
Erik
Technical Support Representative
Needless to say, I tried holding the mode button in SLAVE and it switched to Manual just like you said.