Creating a custom cell symbol for datagrid is very easy. Every cell gets a function, setValue and setSize. When the grid is built, resized or value is changed, these functions will be called.
Message Board is a datagrid sample application that comes with the The Macromedia MX Developer Resource Kit in which you can send and receive message like a email application.
In that sample they have created a custom cell symbol for showing indicating new messages. The following code is used for building the custom cell component
[UPDATE] included the ActionScript 3 version as well 🙂 It became lot more simpler with the regular expressions
Here setValue function is wrongly mentioned as setLabel function. Find and replace 'setLabel' with 'setValue' to make it work.