Recover from failover

Hello list,

May be this information is inside the documentation, but couldn't find it.

I'm doing some tests with postgres 9.1.1 and replication, and I was
wandering if there is some information about how to recover from a
failover.

Suppose I'm doing some kernel upgrades on the master, so in order to
avoid downtime, I promote the slave to become the master, and reboot
the old-master. Once the old-master is up and running again, I want it
to be the master as it was before. The problem are the updates and
inserts made while it was offline. Does it recovers automagically by
streaming or something? Do you have any procedures/recommendations to
do it?

Cheers,

Martin