Upgrading to the newest version of the mongo driver (1.8.1) I get the following exception when starting my Rails app:

/Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongo-1.8.1/lib/mongo/mongo_replica_set_client.rb:116:in `initialize': Too many arguments (Mongo::MongoArgumentError)
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongo-1.8.1/lib/mongo/legacy.rb:71:in `initialize'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config/replset_database.rb:22:in `new'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config/replset_database.rb:22:in `configure'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:288:in `configure_databases'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:305:in `block (2 levels) in configure_extras'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:304:in `tap'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:304:in `block in configure_extras'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:303:in `each'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:303:in `inject'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:303:in `configure_extras'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:112:in `from_hash'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:126:in `block in load!'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:125:in `tap'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/config.rb:125:in `load!'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid.rb:148:in `load!'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/mongoid-2.5.1/lib/mongoid/railtie.rb:84:in `block in <class:Railtie>'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/initializable.rb:30:in `instance_exec'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/initializable.rb:30:in `run'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/initializable.rb:55:in `block in run_initializers'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/initializable.rb:54:in `each'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/initializable.rb:54:in `run_initializers'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/application.rb:136:in `initialize!'
from /Users/fredrik/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails/railtie/configurable.rb:30:in `method_missing'