Yes, later Volvo wagons do have aluminum tailgates. However, they are not 240s, all of which were steel. As far as I know all 7 and 9 series wagons have aluminum tailgates. I'm not sure about the 850/V70, but I wouldn't be surprised if they did too. The main reason for this was to save weight, in the interest of fuel efficiency. Note that the 745 is considerably roomier than the 245, but weighs about the same.

I have heard rumors of aluminum HOODS for both 2 and 7 series. (an R-Sport part, I believe) Perhaps these got confused with the tailgates and the garbled result is the story you heard.

Note to Rusteebrick: Aluminum is by no means immune to corrosion. Road salt and other alkalis attack it readily. I have an 87 745 that has a fairly large area of corrosion under the paint around the rear wiper pivot. It resembles nasty surface rust, and scrapes off easily to leave badly pitted metal.

I have heard a couple of stories of rear bumpers falling off of 740s in far northern climates due to road salt eating away at the (1/4"+ thick) aluminum to the point where the mounting bolts no longer had anything to hold onto.