Text search has been made official as of version 2.6 of mongodb and it appears that it's almost possible to use it by building a Criteria as follows:
Model.where(
:$text => {:$search => "search string", :$language => "en"}
)

However the ruby driver is lacking an api to set the $meta flag on projection field (second argument of db.collection.find(...)), which required to sort by the search score.

http://docs.mongodb.org/manual/reference/operator/query/text/
http://docs.mongodb.org/manual/reference/operator/projection/meta/

It's also hard to say if the Criteria sort method will work properly without being able to test passing along the projection argument.