See the thread starting at:

http://mail.python.org/pipermail/python-dev/2009-October/092958.html

Eric suggested that we don't need a separate branch for this; sounds
fine to me.  It should still be possible to do the backport in stages,
though.  Something like the following?

(1) Check in David Gay's code plus necessary build changes,
configuration steps, etc;  conversions still use the old code.

(2) Switch to using the new code for float -> string (str, repr, float
formatting) and string -> float conversions (float, complex
constructors, numeric literals in Python code).  [Substeps?]

(3) Fix up builtin round function to use the new code.

(4) Make any necessary fixes to the documentation.  (Raymond, I assume
you'll take care of the whatsnew changes when the time comes?)

(1), (3) and (4) should be straightforward.  (2) is where most of the
work is, I think.  I think it should be possible to do the stage (2)
work in pieces without breaking too much.