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This says that it's just tucked away in a lower menu:
How to enable USB mass storage on Android 4.0 ICS Samsung Galaxy S2?
I'll find out in a few minutes - upgrading to 4.03 as I type.
If it's really gone there's always this I suppose:
Alt Mounter brings Android USB Mass Storage back to OS X | Android Community
.
I wonder if Google is going to fix this.