We have just retuned home tonite from WDW. The smoke was quite noticeable this morning- especially at Epcot. However, it was not enough (IMHO) to close any park.
There have been 2 nearby fires this week- one (6 acres) which came right up to the Epcot parking lot. (where it was placed under control.)
The current smoke problem is from a fire which now encompasses over 400 acres, but is about 4 miles from WDW. We could see the red glow in the sky last night. The smoke is very dependant on the wind direction.
Keep in mind that WDW has closed parks only on 1 occasion and that was 2 years ago for Hurricane Floyd. (Even then- AK was kept open for onsite guests.)
I'm sure if, in the unlikely event of all parks being closed, that Disney would do something appropriate to accommodate onsite guests, but most likely- the parks will remain open and as active as ever. It would take a very special set of circumstances for smoke from this current fire to affect all parks at the same time.
Have a great trip!