this works for me with 2.0.0.rc.8 but fails with 2.0.0

irb> e=Event.new
irb> e.startdate="2011-03-03"
irb> e.save
Mongoid::Errors::Validations: Validation failed - Startdate is invalid.

field :startdate, :type => Date

even this fails:

irb> e.startdate=Date.parse("2011-03-02")
irb> e.save!
Mongoid::Errors::Validations: Validation failed - Startdate is invalid.