Quoting IDID-VBAC 12/09:" I overpacked when I flew with my 2.5 month old. She slept the whole time and only needed like 2 diaper ... [snip!] ... like 2 diaper changes. It is hard to not want to overpack but it is annoying carrying things that dont need to be carried."
That's why I did a big bag in the over head withthe "What if" items and a small bag with just one or two diapers and and extra sleep and play to keep down under the seat. Then I just popped the little bag in the big bag when we werent on the plane.