The other day when editing a list in the datasheet view in MOSS 2007 I got "The selected cells are read-only" error. I found a few posts that indicated that not all of a lists data types are supported in datasheet view, but that wasn't my problem because I only had single line text, number and currency data types.
The problem was that I needed folders in my list and although I could edit the datasheet at the root level, I got the read-only error whenever I tried to edit the datasheet within a folder.
On one of my VPC images I had no problems and then I spotted the little Access icon in the top left hand corner of the datasheet. Comparing this to the VPC image where I was getting the error I noticed that the Access icon was missing. On this VPC Office 2007 was installed EXCEPT for Access. So I just installed Access and the error went away.
Please note that the datasheet view does require the Office Web Components to be installed on the CLIENTS for datasheet view to work at all. What I haven't checked yet is whether clients needs Access 2007 in order to edit cells when in a folder of a list. When I find out I'll update this post.