Creating an indexes can be a potentially huge operation. Mongoid should have a configuration to allow people to set it to not automatically create the indexes.

autocreate_indexes: false

An associated work item for this would be to provide a Rake task (or simple Mongoid helper like Mongoid.setup_indexes) that someone can run to create the indexes outside the scope of running their models within an app.

From this thread:
http://groups.google.com/group/mongoid/browse_thread/thread/445a5683bc8e19e6