This is treated extensively in the discussion section of the iterators-PEP; quoting:

- It has been questioned whether an exception to signal the end of the iteration isn't too expensive. Several alternatives for the StopIteration exception have been proposed: a special value End to signal the end, a function end() to test whether the iterator is finished, even reusing the IndexError exception.

- A special value has the problem that if a sequence ever contains that special value, a loop over that sequence will end prematurely without any warning. If the experience with null-terminated C strings hasn't taught us the problems this can cause, imagine the trouble a Python introspection tool would have iterating over a list of all built-in names, assuming that the special End value was a built-in name!

- Calling an end() function would require two calls per iteration. Two calls is much more expensive than one call plus a test for an exception. Especially the time-critical for loop can test very cheaply for an exception.

- Reusing IndexError can cause confusion because it can be a genuine error, which would be masked by ending the loop prematurely.

I'm not sure why you are reopening this -- special terminating values are evil IMO. :-)

--Guido van Rossum (home page: http://www.python.org/%7Eguido/)