In the Southern Hemisphere (Brazil, Australia, South Africa, etc.) in October they are heading into the summer. Do they say Mashiv HaRuach at this time of the year or do they say it from Pesach to Succos?
All of the discussion on this seems to go back to Shaalos Uteshuvos Toras Chaim (by R' Chaim Shabsi, 17th century, in response to a halachic query - the first known one, by the way - from the Jewish community in Brazil). He says that during the times when they need rain, if it's not during the Northern Hemisphere winter, they should just request it (i.e., say ותן טל ומטר, and I guess also משיב הרוח) in the blessing שומע תפלה; otherwise they should omit it altogether. In short, then, there might indeed never be a time when they say it.
"Mashiv haRuach" is not a seasonal request, so I would wonder about a printing error in those commentaries that put it in those terms.
"veSeein Tal uMatar" on the other hand is a seasonal request , and the timing used is specific to the region around Eretz Yisroel. So the question arises as to why do others start saying it at the same time (@ Dec 04). In fact, the ROSH among others , tried to change the starting time to be appropriate (sp?) to the region one is living. Obviously they lost the argument, what is not so obvious is why. It has been suggested that the Eretz Yisroel time was kept as a type of "Zecher l'Churban".