ObjectiveResource
Pretty cool:
ObjectiveResource is an Objective-C port of Ruby on Rails’ ActiveResource. It provides a way to serialize objects to and from Rails’ standard RESTful web-services (via XML or JSON) and handles much of the complexity involved with invoking web-services of any language from the iPhone.
I’ve been considering trying out some iPhone development (even though I don’t yet own one), and this seems like a really slick way to interface with an existing Rails application.