We all know that SharePoint has built-in support for "Append-only comments". However, it needs to turn on the versioning feature, which could waste disk space.
Another normal choice is to purchase or to build custom field type to get that functionality. It would cost more time and money, and it brings more trouble in SharePoint 2010. SharePoint 2010 built-in InfoPath forms don't support custom field type!
The last choice is my favourite: implement "Append-only" functionality through one extra column and a list workflow. Below is the example. If we want, we can change the field "Comment History" to Read-Only, or just customize the edit form and view form through InfoPath.
Here are some relevant screenshots:
Update (26/07/2010): If you want to get the user full name instead of user login account, please check Useful Sharepoint Designer Custom Workflow Activities. The user full name property is "LinkTitle", and the user email address property is "E-Mail". For SharePoint 2010, we need to install this solution manually through "stsadm" command or powershell.