I am looking to bid on a project that would require wireless in Antarctica. The temperature there can get as cold at -90 degrees Fahrenheit. Can Mikrotik's handle that kind of cold?

-90°F =~ -68°C
Not.
The range are from -30° to +60° Celsius

But ask sales@mikrotik.com

If you don't put it outdoors it would.

Actually RB800 and the RB Metal could handle it, if you start it warm. If it is frozen for a long time, then it would have trouble booting, but once it has started, it will keep itself warm. We have a temperature chamber, we could do some tests for you once you have chosen a model.

The temperature range is usually quite standard, we rarely test outside the range.

The problem with very low temperatures are not necessary immediate.
Tin changes its crystalline structure over time at low temperatures, becoming brittle (actually a grey powder):
http://en.wikipedia.org/wiki/Tin_pest
This could be an issue, especially with the wonderful RoHS alloys not containing Lead.