Housekeeping does accept tips. As a PP mentioned, some resort rooms even have Disney-prepared tip envelopes laid out, to make tipping easier. This was the case for us at the Grand Floridian and the Yacht Club last summer, for example. And when we forgot to tip before check-out once at the Polynesian, after check-out we went to the front desk to leave the tip for our specific housekeeper. They (management who sent the tip on to the housekeeper) understood & appreciated it & did not comment in any way that tipping housekeepers is not customary at Disney.
It is customary nationwide, although a lot of hotel guests aren't aware, because people stay at hotels less frequently than they eat at restaurants. More experienced travelers tend to be more familiar with this etiquette rule.