I would not go up to a celeb at Disney as much as I wuold probably want to. It's their off time and one little picture could lead to them being SWARMED.
That said, I had lunch with the oldest son from Home Improvement and his family and we did take a picture with him. What happened was, we were at ESPN cafe in boardwalk the day it opened and it was just a clusterXXXX all around. We waited over 2 hours to get our lunch. They were at the table next to us and we just bonded. We didn't even realize it was him until he said he was there for a disney promotion (all the ABC stars of the Friday night line up were there at the time). When we finally got our food and ate, we asked if we could take a picture.
That same week we saw Corbin Bernson on Tower of Terror and all the kids from Boy Meets World.
Another time, we saw Evander Hollyfield and his family coming out of The Living Seas. This was right after the Mike Tyson incident and I sooo wanted to make a joke but I was a good girl.