Identity Map and Domain Objects

Bit of an abstract problem here. I’m experimenting with the Domain Model pattern, and barring my other tussles with dependencies - I need some advice on generating Identity for use in an Identity Map.

In most examples for the Data Mapper pattern I’ve seen (including the one outlined in this book: - the user appears to manually set the identity for a given Domain Object using a setter:

$UserMapper = new UserMapper;

//returns a fully formed user object from record sets
$User = $UserMapper->find(1);

//returns an empty object with appropriate properties for completion
$UserBlank = $UserMapper->get();

Now, I don’t know if I’m reading the examples wrong - but in the first $User object, the $id property is retrieved from the data store (I’m assuming $id represents a row id). In the latter case, however, how can you set the $id for an object if it has not yet acquired one from the data store?
The problem is generating a valid “identity” for the object so that it can be maintained via an Identity Map - so generating an arbitrary integer doesn’t solve it.

My current thinking is to nominate different fields for identity (i.e. email) and demanding their presence in generating blank Domain Objects. Alternatively, demanding all objects be fully formed, and using all properties as their identity…hardly efficient.

(Or alternatively, dump the Domain Model concept and return to DBAL/DAO/Transaction Scripts…which is seeming increasingly elegant compared to the ORM implementations I’ve seen…)