I think Alexander's example is best written as:
"Today is: {0:%a %b %d %H:%M:%S %Y}".format(datetime.now())

I agree with Brett that there's nothing unusual about having type-specific formatting languages, and for datetime strftime is the obvious choice. It's a little unfortunate that % is used, as it's mildly confusing with %-formatting, but not much can be done with that issue.