I'l blatently steal Bob Pullen's title and say At mx.Last
You've been asking for this for years and we've been looking at it for nearly as long. We promised recently that we'd deliver and now we have.
We've just rolled out some changes to our Domain Control System that now allows customers to remove the secondary MX record (mx.last.plus.net) from domains they have hosted on their account.
This was the 5th most voted for network item in the Usergroup's Issue Tracker so is something customers have been asking about for a while.
Bob has written a fantastic blog post about this here
http://community.plus.net/comms/2007/07/30/at-mxlast/Any feeedback or comments will be greatly appreciated but I'm sure you'll join me in celebrating this peice of welcome news.