The schema design depends on chatrooms/users/messages numbers. I.e. you can have one CF, where key is chatroom, column name is username, column value is the message and message time is the same as column timestamp.
You can add day-timestamp to the chatroom name to avoid large rows.

Augi

2011/2/15 Andrey V. Panov <panov.andy@gmail.com>
I never did it. But I suppose you can use "chatroom name" as key and store messages & nicks as columns in JSON and timestamp as columnName.