Gregory-Aland P67 (P. Barcelona 1) is the oldest manuscript that contains Matt 3:15. What’s the oldest manuscript that contains Matt 3:16-17?
UPDATE (Dec. 4, 2006, 9:20 a.m. EST): No one got it yet.
HINT: The answer is not to be found in the Kurzgefaßte Liste (nor even in its online continuation Eric Rowe pointed to in the comments [thanks, Eric]).
FURTHER HINT (Dec. 4, 2006, 5:35 p.m. EST): The manuscript I have in mind is indeed listed in the Leuven Database of Ancient Books, but it is not a manuscript of Matthew.
ANSWER: Andrew Criddle and Eric Rowe teamed up for joint credit, earning a free subscription to Hypotyposeis. See Matt 3:16-17 in POxy 3.405 (Dec. 4, 2006) for more information about the answer.