When you run the CVS commit command in MacCVSClient, one single CVS commit request is sent to the CVS server. This request gets all selected files and folders as arguments. So, on this high level, one could say that commits are kind of atomic.

If you are thinking about this in terms of short update log emails with combined entries for groups of files that have been committed togther, then yes, MacCVSClient does what you want.

On the other hand, if the question is whether the CVS server implements technically pure atomic commits, the answer is no. At this moment (CVS server 1.12), commits are not atomic. This means: when commiting a group of files with one CVS commit request, it can happen that some of the files are committed, while others aren't. This is a known issue on the server side and cannot be addressed by client software.