I would like to work on a vineyard in Argentina or Chile. I can only
stay for a couple of weeks (2-4 weeks). I could start work in the time
frame of December 2006 - March 2007.

I am looking for this kind of work: land preparation, planting grapes,
nurturing, irrigating and harvesting them, to crushing the grapes in
wineries and turning them into wine.

Are there any vineyards that are looking for help? Is there any web
site or book that lists vineyards that have these kinds of jobs on a
regular basis?

Vines do not grow in 2-4 weeks

The only seriously labour intensive work is picking the grapes
- and believe me that is pretty back breaking.

Yes I know, but at least I can learn part of the process and I don't
mind the back breaking :-)

Hi, i am from Argentina.
I can provide you a list of vineyards so you can put in contact with
them, and apply for the job you are looking for. Most websites are in
Spanish (if you need further help with language, tell me).

http://www.viarural.com.ar/viarural.com.ar/agroindustria/bodegas-y-vinedos/default.htm

Here, for example, you can see the different vineyards, in the
different provinces of Argentina (San Juan , La Rioja, Mendoza, Buenos
Aires, and others). The information on each vineyard is listed there,
so you can contact them.

Wish it helps!
Good luck