Is automatic failback on Postgres 8.4 possible via log shipping/PITR?
In general, no, not possible to turn the original master into a standby of the new master without making a copy of the new master, replacing all the data files of the old master with said copy, and resuming log shipping.

This is because, in general, there will be lost transactions on the old master that didn't make it to the standby.

You could use rsync to make the copy go faster -- as the contents of the two databases will likely be very similar. The tool pg_rewind was invented for this purpose: it will "rewind" the old master to the point at which the failover happened.

See Heikki Linnakangas' excellent presentation about the whole topic at http://hlinnaka.iki.fi/presentations/NordicPGDay2015-pg_rewind.pdf