MVC 3 relies on the jQuery Validation plugin for client side validation. You need to configure the plugin to not validate on key up.
You can switch it globally off using
$.validator.setDefaults({
onkeyup: false
})
See http://docs.jquery.com/Plugins/Validation/Validator/setDefaults and the onkeyup option here http://docs.jquery.com/Plugins/Validation/validate.