We created a lot of noise with the original 100 degrees Celsius article. You can read it here.
Luckily we have friends who wanted to help us out in our quest for truth. It turns out that testing the card without a case on in a 20 degrees C environment, the temperature of the Radeon HD 2900 XT will reach over 82 degrees.
There is a catch in 2D mode, as the temperature will be stable at about 58 degrees. In 3D mode the R600 chip heats up to an average of 68.4 degrees or in worst case scenario 71.3 degrees.
In a closed case enviroment the situation gets even worse, so you can expect even more than 82 degrees, not far off the 100 degrees that we originally reported.
Here is a picture.