Streaming Replication 9.2

I've setup streaming replication between two 9.2 servers, and have a few concerns/questions. I set it up with a very large wal_keep_segments, 17000, and do NOT ship the logs to the standby.

When I failover to the slave, MUST the process for bringing back up the former master initially as a slave involve copying a base backup from the new master back to the former master? Even if I am keeping enough wal segments to include all changes since the former slave was promoted? I tried just bringing up the former master as a slave but it complains "timeline 2 of the primary does not match recovery target timeline 1".

If this is true, what is a good strategy for handling primary/slave in distant geographic locations where copying a base backup between the two is not terribly convenient?