FCC part 15 "must accept interference from other sources": what does this mean?
FCC Title 47, Part 15 section 5.b should clarify this for you: 


  (b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions ... that interference must be accepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiator, by industrial, scientific and medical (ISM) equipment, or by an incidental radiator.


In other words, to be FCC listed under part 15, your device must operate normally when subject to normal, authorized RF emissions from other sources.

You can drill down to the specific requirements under part 15 by starting here:

http://www.fcc.gov/encyclopedia/rules-regulations-title-47