There is a Johns Hopkins Applied Physics Lab report which stated that there needd to be more testing done to determine a safe distance from the machines. They did some analysis which recommended that a safe distance, pending further testing, was something like 14' above the machine (i.e.: the next floor above in most cases), 6' behind, and 4' to either side because they couldn't determine if radiation levels within these distances was within safe levels. Radiation follows the inverse square law. For example, the radiation 8' from either side would be 1/4 of what it would be at 4' away.

In other words, give yourself a wide berth and err on the conservative side while the screeners happily x-ray passengers, blissfully ignorant.