The pcl-cvs program has historically had an odd way of dealing with error and informational messages from CVS (although this may be corrected in the latest versions). When it encounters a message from CVS that it doesn't know about, it gets hysterical and throws you into a mail buffer, ready to send a pregenerated bug report to the author of pcl-cvs. Unfortunately, among the CVS messages that pcl-cvs may not know about are the ones associated with conflicting merges, which, although not common, certainly do occur from time to time.
If pcl-cvs suddenly dumps you into a mail buffer, don't panic. Read over the contents of the buffer carefully – the offending CVS output should be in there somewhere. If it looks like a merge, you can just get rid of the mail buffer and rerun cvs-update. It should now succeed, because CVS won't output any merge messages (because the merge has already taken place).
(Update: this problem appears to have been fixed in more recent versions of pcl-cvs, so very probably you can ignore this entire warning.)