Can I use FluentValidation for validating untyped data? (Datasets, NameValueCollections, FormCollection etc)
FluentValidation is for validating typed properties on objects. Whilst it's technically possible to validate untyped data, you'd be better off using something else.
I'm using ASP.NET MVC, and my model has non-nullable value types. Why don't all my rules run at the same time?
This is one of many limitations of ASP.NET MVC's validation infrastructure. It does not allow model-level rules to be run until all non-nullables have a value specified.
Will you add integration points for WPF/Silverlight/Some other UI tech?
No. The core FluentValidation library runs just fine in WPF/SL projects, but there will not be any specific integration points provided for different UI technologies.