Wow! What a timely thread. Today is my DD's 4th birthday and last year we were at WDW for her 3rd b-day! It was the last day of our trip, and I had been worried about the same thing. I asked on the Mom's Board on the WDW website and they told me that if she's 2 at the beginning of the trip, she's considered 2 the entire trip. So, that being said, I was only questioned once about her age, but she isn't tall for her age (when we went to Sea World last May, they questioned me why I handed them 2 tickets - they thought she was still 2). When I was questioned about where her ticket was, I simply replied 'she's 2' - and they let us move along.
At WDW, we did the lunch at Cinderella's castle a few days before and put her birthday on the reservation. They didn't charge us for her, brought her a plate of pasta w/ butter on it and milk (to drink), and a special little birthday cake-for-one (as well as a scoop of ice cream). It was awesome! It made the overpriced meal not feel so overpriced

!
So, I would definitely take advantage of this one last 'free' trip - and maybe even sprinkle some extra birthday pixie dust on it!
Also, we got tired of worrying about misplacing the birth certificates, so we went to our local MVD and got the girls both official ID cards. I never have to carry their BC's and I always have proof of their age, if needed.