I would agree with Marc that a DVWP would be better in this case. I understand it is not an option for you.
Creating the button is the easy part, the problem is the onclick event handler.
Marc, the amount of script is not necessarily an issue, you can always put it in a function that will be called by the button.
The main issue with calculated columns is that you cannot reference the item ID in the formula. You need a workflow trick to copy the ID to a standard field.
The bottom line is that it is not too difficult, but involves multiple steps/workarounds:
- workflow to copy the ID
- calculated column that creates the formula
- text to HTML to display the button
- externalize the onclick event handler in a function that calls the Web service to update the item.
- and of course error handling along the way
Are you on SP 2007 or 2010?