We're starting a new project, on the client side, we'll use a asp.net mvc3 application, accessing resources exposed by OpenRasta.
asp.net mvc has built in validation with Data Annotations, how usually people do validation with OpenRasta?
It depends the kind of validation you want. We don't currently support anything out of the box because the scenarios of what people wanted has always been very nebulous. People usually apply validation on the constructed model themselves rather than get OW to do it.
Happy to discuss your requirements and implement them as needed in 2.1, there's still a small open window for certain new features.