Not yet, I've been trying to think of an elegant way of handling exceptions. In a lot (most?) of cases you won't want to return a stack trace to the client; instead a simple error code/message would be more useful. Whether this is something that is done client side or server side I'm not sure. I'm leaning toward server side because it would be nice to support I18N for error messaging, and this way your error messages could be stored in a message bundle resource.
As for how the message is handled on the client, I was thinking something along the lines of allowing exception handlers to be registered at the method level, e.g. Seam.Remoting.registerExceptionHandler("componentName", "componentMethod", handlerMethod). Then your exception handler could do something graceful, like display a 'Sorry the customer details could not be retrieved at this time' message, etc.
Ok. It is funky idea to have a seperatate callback on error. I think MochiKit allowed you to do this. I don't understand completely what you want to do but what is important to me is that I could still provide a message with my exception and have that displayed to the user. eg..
public BigDecimal getBalance(String accountNumber) throws MyRemoteException
// not a valid account
throw new MyRemoteException("You have entered an invalid account number.");
// account closed
throw new MyRemoteException("This account is closed.");
It may be cleaner to pass error codes back in the exception and then match that to a localized message but I don't want to be forced to do that.