The selected cells are read-only

Today I received a report from a user that while using Datasheet View to manage a list they were presented with the following error message:

"The selected cells are read-only"

I have come across this in the past and the solution was to disable content approval for the list, as Datasheet view does not support content approval, but in this case the list was not set to require content approval.  After a bit of playing around with the column settings I determined that changing the type of text to allow from Rich text to Plan text solved the problem.