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> Do you have a real use case for this?

For the 'hopefully bugs' variants, yes:

#1: Loop forever:

  for i in xrange(x, sys.maxint, y)

That's a lot faster than

  i = x
  while True: ... i += y

#2: 'loop until optional upper bound':

  def some_loop(start, end = sys.maxint):
    for i in xrange(start, end, whatever())

> Do any real apps need to loop over more than
> sys.maxint integers?

The answer may be yes nowadays.  Even my old
Solaris can find primes up to maxint/2 in just
2 hours.  That's a loop over maxint/4 integers.
Though the remaining 3/4 come slower:-)

Still, I expect variants of the above code would
be less uncommon, like some_loop(-100).

> It would be ashamed to muckup the high
> performance implementation for something that
> does not arise in practice.

I hope you do not mean xrange(0, maxint, 3).

If you mean xrange(-100, maxint): Maybe xrange
could be split in several types (fast and slower)
and the xrange() operator would return one of
these, similar to how int() now can return long?