The glacier walks run year round, weather permitting. I did a heli-hike on
the Franz Joseph Glacier (Sept 2004), and my guide said that the weather is
clearer in winter. He also said that the glacier is easier to walk when
it's covered in snow (it's easier to walk on snow as opposed to ice). You
are required to wear crampons year round (supplied by the tour company), but
the guides seen to think that they cover more ground on the glaciers in
winter as opposed to summer. Don't worry about traveling during winter. My
tour of the South Island was from Mid-Aug. to End-Oct. 2004, the weather was
never a problem (tracks covered in knee deep snow is a different story ~:-).
Check with DOC (preferably) or the Info Center before heading out on a hike.
Weather forcasts are reliably unreliable (just like anywhere else on the
planet).

Enjoy!

Dwayne