If I try to configure a replicaset, I get this error with v1.8.4 (released to Rubygems yesterday):

It appears that :reader_preferred is no longer a valid read preference (code is expecting a hash now), but is being passed on lib/mongo/db.rb:126

/Users/asalomon/.rvm/gems/ruby-1.9.3-p385@mash/gems/mongo-1.8.4/lib/mongo/util/read_preference.rb:47:in `merge': can't convert Symbol into Hash (TypeError)
from /Users/asalomon/.rvm/gems/ruby-1.9.3-p385@mash/gems/mongo-1.8.4/lib/mongo/util/read_preference.rb:47:in `read_pool'
from /Users/asalomon/.rvm/gems/ruby-1.9.3-p385@mash/gems/mongo-1.8.4/lib/mongo/mongo_replica_set_client.rb:342:in `block in checkout_reader'
from /Users/asalomon/.rvm/gems/ruby-1.9.3-p385@mash/gems/mongo-1.8.4/lib/mongo/mongo_replica_set_client.rb:326:in `checkout'
from /Users/asalomon/.rvm/gems/ruby-1.9.3-p385@mash/gems/mongo-1.8.4/lib/mongo/mongo_replica_set_client.rb:341:in `checkout_reader'
from /Users/asalomon/.rvm/gems/ruby-1.9.3-p385@mash/gems/mongo-1.8.4/lib/mongo/db.rb:126:in `authenticate'