Is there a safe temperature definition for the Macbook Pro Retina? I'm using a grid-computing software to donate my CPU time, so every core is always 100 Celsius degrees (212F), all day.

Is there a reference, from Apple or other source, indicating a safe temperature for this notebook?
According to the intel, spec-sheet, Page 20, the maximum allowed temperature of all the new i3,i5,i7 cpus is 105 Celsius degrees so you are approaching the limit. However all cpus are designed in a way that they will send a signal to the motherboard in case the cpu is overheating to either shut-down the pc completely or dramatically reduce the performance (by lowering the clock-speed). 

Studying Anandtech's review of the latest MacBook Retina it's clearly visible that the Macbook pro retina automatically reduces the performance in case of really high temperature over a longer period of time. According to them while running various benchmarks in Half-Life 2 the performance of the retina model dropped by about 5% compared to the old model which dropped by about 20%). 

I think you don't need to worry to much about the temperature. If the cpu is reaching its thermal limit it will automatically try to do something about it. You may even think about investing in a cooling pad as the enclosure of the mac is made out of aluminium, it acts a a massive heat-sink and theoretically you could lower the temperature by  using a cool-pad for laptops and fit it underneath.