Same here, in New York, since last night (Friday 11th early morning).
All my iCloud synced devices are affected (iMac, Mac Book, iPad), but noy my external HD running Snow Leopard and, thus, not iCloud synced. Oddly enough, while I was able to send mail while under Snow Leopard, I received a warning about the security certificate, asking me whether I wanted to trust it anyway.
If I go into iCloud online web, I can send mail from there, but not from my Mail app.
My wife's Mail account uses p01 instead of p04 as smtp server (I don't know how servers are assigned by default when setting up a Mail account) and she's experiencing none of these issues.
I have spent the best part of the morning chatting to Apple support. After going through all the motions and showing them this very discussion, now they are looking at it, or so they tell me.
I am sill online with an advisor.
This is proving very, VERY frustrating.