As I’m still learning ASP.NET MVC, my solution may not be the optimal one.

Create a custom model binder that will only react differently to double values.

using System;
using System.Globalization;
using System.Web.Mvc;

namespace NerdDinner.Helpers {
  public class CustomModelBinder : DefaultModelBinder {

    public CustomModelBinder()
      : base() {
    }

    public override object BindModel(ControllerContext controllerContext, 
      ModelBindingContext bindingContext) {
      
      object result = null;

      // Don't do this here!
      // It might do bindingContext.ModelState.AddModelError
      // and there is no RemoveModelError!
      // 
      // result = base.BindModel(controllerContext, bindingContext);

      if (bindingContext.ModelType == typeof(double)) {

        string modelName = bindingContext.ModelName;
        string attemptedValue = bindingContext.ValueProvider[modelName].AttemptedValue;

        // Depending on cultureinfo the NumberDecimalSeparator can be "," or "."
        // Both "." and "," should be accepted, but aren't.
        string wantedSeperator = NumberFormatInfo.CurrentInfo.NumberDecimalSeparator;
        string alternateSeperator = (wantedSeperator == "," ? "." : ",");

        if (attemptedValue.IndexOf(wantedSeperator) == -1 
          && attemptedValue.IndexOf(alternateSeperator) != -1) {
          attemptedValue = attemptedValue.Replace(alternateSeperator, wantedSeperator);
        }

        try {
          result = double.Parse(attemptedValue, NumberStyles.Any);
        }
        catch (FormatException e) {
          bindingContext.ModelState.AddModelError(modelName, e);
        }

      }
      else {
        result = base.BindModel(controllerContext, bindingContext);
      }

      return result;
    }
  }
}

Change the DefaultBinder in Global.asax.

void Application_Start() {
  //[SNIP]
  ModelBinders.Binders.DefaultBinder = new CustomModelBinder();
}