Hi Adam,
Nokia says 6230 supports at least 256mb but that's for an official Nokia MMC card MU-9:
http://www.europe.nokia.com/nokia/0,5184,5987,00.h
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Even 1GB has been known to work but I've seen on other forums that people have had problems after they've formatted a card that's 512MB or bigger using their phone.
So can you try formatting the MMC in a MMC reader instead of the phone and then try again?
All those who believe in psychokinesis raise my hand.