Prior to recent changes (I think I was on something in between 0.7 and 0.8 I was able to create an index using:

coll.create_index([['users.email', XGen::Mongo::ASCENDING]])

On mongo-ruby-driver HEAD (71d7ff726b38f440d7df47b6d39cda3636370bfc) I get the following error:
/var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:108:in `serialize_key_value': key users.email must not contain '.' (RuntimeError)
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:94:in `serialize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/ordered_hash.rb:50:in `each'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/ordered_hash.rb:50:in `each'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:94:in `serialize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:410:in `serialize_object_element'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:117:in `serialize_key_value'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:94:in `serialize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:94:in `each'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/util/bson.rb:94:in `serialize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/message/message.rb:62:in `write_doc'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/message/insert_message.rb:30:in `initialize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/message/insert_message.rb:30:in `each'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/message/insert_message.rb:30:in `initialize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/db.rb:476:in `new'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/db.rb:476:in `create_index'
from /usr/lib/ruby/1.8/mutex_m.rb:67:in `synchronize'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/db.rb:475:in `create_index'
from /var/lib/gems/1.8/gems/mongo-0.8/lib/mongo/collection.rb:132:in `create_index'

Is there an alternate syntax for creating indexes on embedded document fields or is this a bug..?