Most tupperware should be "dishwasher safe", meaning it's been tested to high temperatures and won't melt even in the lower rack of the dishwasher.

I think there is vocabulary confusion happening here.

Real Tupperware -- the expensive stuff -- is nigh-indestructable. Some of it is made out of polycarbonate, the same material used for windshields in fighter jets and in presidential limos. At the thickness used in the Tupperware line, it is not quite bulletproof, but it is still very, very tough. You don't have to worry about it in the dishwasher.

Lower-end Rubbermaid plastic containers are much cheaper and not made out of the same material. (Rubbermaid does have a "premier" line that is supposedly comparable to true Tupperware.) These bins should not be placed in the lower rack of the dishwasher.