I have not been on a WISH trip, but have been at WDW during that time period. Until January 1, the crowds will be so large that even at Epcot (the park with the widest and least congested walkways), you will literally not be able to put your arms out without touching someone else. At the smaller parks, I felt like a fish in a school of fish. We literally had to "go with the flow" at times and just go where the crowd was going. Since you are on a WISH trip, you will have expedited waits, but waits for things other than attraction (like bathrooms and counter service restaurants) will be long and congested.
January 1 was less busy because many people had been out late the night before and did not go to a park early. By January 2, most of the holiday crowds had left and we had a much more pleasant time.
That may not be the case this year; since New Years Eve is on a Thursday for 2009, I expect most guests would stay the whole weekend and leave either on Sunday, January 3rd or Monday, January 4th.
If you have not already seen it, I am going to suggest you look in post #3 of the disABILITIES FAQs thread. That thread is stuck near the top of this board, or you can follow the link in my signature to get there.
Post #3 of that thread has a lot of helpful information, including a section about WISH trips. That section includes a link to a long thread on the Community Board called WISH Trippers Unite. You will find a lot of people who have been on or are planning WISH trips hang out there. You may even be able to find someone who had a trip in the same time frame as you are planning.