http://api.mongodb.org/ruby/current/Mongo/ReplSetConnection.html has the following example of code.

ReplSetConnection.new(['localhost:30000', 'localhost:30001'], :read => :secondary)

I create a local replica set with 2 nodes listening on the ports listed above.

Calling this will fail with a message: Mongo::ConnectionFailure: Cannot connect to a replica set using seeds localhost:30000:localhost:30001

Changing it to Mongo::ReplSetConnection.new(['localhost',30000], ['localhost',30001], :read => :secondary) does work correctly.