This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
If you know, before sending the response to the client, that you're going to set the readonly attribute then you can do this with server side code. If it needs to change according to the user's input, without sending another post to the server, then this will need to be done on the client.