Hi,
i submitted a patch a week ago to support validation messages in json for ActiveResource. Currently, only xml are supported and the xml parser is called even if response is in json format.
Ticket is at #1956 [PATCH] json validations errors for ActiveResource - Ruby on Rails - rails
and patch itself : http://rails.lighthouseapp.com/attachments/88393/json_validations_for_active_resource.diff
Thanks.