Streaming replication failover

I'm in the process of setting up a 9.1-based SR cluster, and I've got a question on how failover is expected to work in the case of multiple slaves. http://www.postgresql.org/docs/9.1/static/warm-standby-failover.html says:

"Some people choose to use a third server to provide backup for the new primary until the new standby server is recreated, though clearly this complicates the system configuration and operational processes."

I think a third server sounds like a swell idea, but I'm unclear how a slave based on the old master can act as a slave for the new master. I thought that once the master switched, you'd get a new wal timeline, and any existing slaves won't follow along with the new timeline. Which is really unfortunate, because it means the only way to have a slave, immediately after a failover, is to build one ASAP and hope you don't have insurmountable problems while that's going on. For a big database, that can take a while.

Hopefully I'm missing something?