Google's implementation is peculiar: each collection (calendar) is treated as a separate account. This separation becomes clear after you use
Calaboration to configure iCal.
I might not expect a CalDAV client such as Calendar in iPhone OS 3.0 to work around this type of peculiarity.
To move from one collection to another, without regard for accounts: use iCal in Mac OS X.
To copy from one collection to another: use iCal, with the alt (option) key. The effect is duplication; a change in one copy is not effective in the other.
To have a single item in multiple collections: use
Chandler Desktop.