Since UML has "many modes":http://martinfowler.com/bliki/UmlMode.html you should be more precise about your goals or reasons wanting to use the language.
In general, using UML (in whatever "mode" or abstraction level) for any reason in a Rails project shouldn't be much different from using the same language in any other platform or project. The only way you could make the UML "Rails aware" would be modelling some aspects of the application in a quite low technical abstraction level, like using some special UML profile (or a meta-model) that includes the main Rails concepts. But, modelling an application in that level wouldn't be sensible. Ruby and Rails have clean and elegant syntax that could turn out to be more easier to write, read and maintain than a corresponding UML-model on the same level of abstraction.
The only reason to use UML (or any graphical diagrams or languages) in the context of Rails app development would be marking down some ideas (for communication, on the regular drawing board) of the concepts around the MVC pattern - main models, controllers and views before implementing a feature. It's also important to remember that this "design" would be preliminary design and the final form of the design would take place while writing the code and refactoring (a lot).
Another theoretical possibility of using UML is in the context of the model driven engineering, so that the rails app could be generated from models, but in practice, some specially crafted textual or graphical DSL-s should be more suitable for this task. But I think this isn't the case here :)