I have the exact same problem in that suddenly I can no longer send mail from my mobile me accounts from my macbook air,iPad or iphone. Everything was fine until a day or two ago. Something has changed at the server as I get an error message that says "server rejected sender address."
Most common cause of this problem is the SMTP server is rejecting your messages because your configured from address is somehow incorrect, or using a value that is not allowed for sme reason. Who is rejecting the messages, smtp.me.com or smtp.comcast.net? In other words, what SMTP server are you sending through? What is your configured email address in these clients? What port are you using? SSL or STARTTLS?
My Mobile Me account continues to work fine here from my MacBook Pro, my iPad, and my iPhone. I use smtp.me.com, port 587, SSL on, password authentication.
If you have an issue with your account, you should contact Mobile Me Support, this has nothing to do with Comcast.