ref vs idref attributes in spring bean declaration

[maniganda.prakash]

ref vs idref attributes in spring bean declaration.
Can someone tell me the difference and which one has to be used when ?

<bean id="b1" class="" />

<bean id="" class="">
<property name="b1" ref="b1" />
</bean>

vs

<bean id="" class="">
<property name="b1" idref="b1" />
</bean>

I do have a reference to the following thread, but it doesn't say when to use one over the other.
http://forum.springsource.org/showthread.php?t=74355

[denis.zhdanov]

1. 'idref' is not a valid spring <bean> element attribute. You get corresponding error if you attempt to start the container with such a configuration;
2. The reference is quite cleat about <idref> element usage - 3.3.2.1.1. The idref element;