Hi Jason,
Yes. It does appear to be isolated to iCloud / Mobile Me at the moment.
If I leave $MailFrom = "FromNameHere|WA|from@email.com"; and send it to a simple SMTP server (say something hosted on a linux box with accounts setup through CPanel) then it delivers every time, but with an iCloud address it never arrives until I change it to $MailFrom = "from@email.com";
It used to work up until a couple of months ago, so no doubt Apple changed something!
Cheers
Ian