That's a great question. I've been in the seasons since week one and as I remember it you had to mention the week and the theme at the beginning. During the third week
http://www.youtube.com/watch?v=1fSP_eIMYUg&list=UUxHfQYzIvVLT__m_zRZOhYQ&index=36 when I was pissed about something I just typed the week and theme on a piece of paper and held it up. I think at the start (correct me if I'm wrong Chrimess) that part of the challenge was getting the intro in along with the take (higher level of difficulty.) As it evolved over the next 43 weeks I have seen people mention it at the start of the video or at the end. I have done it at the end once or twice, especially a particularly difficult week with lots of takes, but I usually get so excited to finish the 100th or so take that I end up flubbing that. So, I guess it depends on each host, but I have found that most hosts are cool about it, as long as it's mentioned somewhere in the video, either written or spoken.