Right now the Store interface has no method to open the store. This method is only in SQLStore, which makes the API inconsistent. I suggest adding these methods:

/**
* If store is open, it can be read or written to.
*/
boolean isOpen();

/**
* Opens store in current thread (blocking).
*/
Store<T> open();

/**
* Opens store in background thread and then callback#onSuccess is called.
*/
void open(Callback<Store<T>> callback);

with the last, asynchronous one as optional, probably only if Store<T> gets more async methods. Otherwise I might be confusing for users.