[...]
Hmm, I'm not convinced by these toy examples, but I did inspect some of my own code for incompatibility with the proposed change. I found that there really is only one recurring pattern I use that I'd have to change and that is how I've implemented several file parsers. I tend to write them like this:

def parser (file_object):
    while True:
        title_line = next(file_object) # will terminate after the last record

        try:
            # read and process the rest of the record here
        except StopIteration:
            # this record is incomplete
            raise OSError('Invalid file format')
        yield processed_record

So I'm catching StopIteration raised by the underlying IOWrapper only if it occurs in illegal places (with regard to the file format the parser expects), but not when it indicates the end of a correct file.
I always thought of letting the Error bubble up as a way to keep the parser transparent.
Now in this case, I think, I would have to change this to:

def parser (io_object):
    while True:
        try:
            title_line = next(io_object)
        except StopIteration:
            return
...

which I could certainly do without too much effort, but could this be one of the more widespread sources of incompatibility that Steve imagines ?