Just posted a FAQ "Displaying Custom Messages in Validation Summary" in ExtremeExperts.
Description:
Most of us would have come across a situation where we need to display a custom message other than the error message from validators in the validation summary. For example, based on business logic check you need to display a message to the user. Already you have a validation summary in the page where you are displaying error message to the user, you might prefer to add your message to that validation summary. This faq explains about one of the way to achieve this.