Tweets from today:
http://community.livejournal.com/tohosomnia/435081.html
Junsu replied to fans regarding their questions about the official flashmob choreography
(Si Eun) @0101xiahtic Mr Junsu..You said this would be simple... But this is proper choreography kekekeke [...]
But this is the first time I've seen singers assigning homework to their fans kekekeke You're the best! (4:27pm KST)
(Junsu) @12Faith26
Because this is the first time, if you guys can succeed, we'll probably be setting another 'First' in Asia^^ (4:42pm KST)
LOL, Jaejoong:
(Jaejoong) To those who think the choreography is too hard, try learning the hand movements separately ^^ And I want to see all of you doing the wave together ㅠ If you can't learn the moves, the wave will come...to nothing ...ㅠ (11:32pm KST)
(Jaejoong) I've heard rumors that some people, who feel embarrassed about dancing, are going to "have a glass of soju" before coming..haha Don't do that (11:34pm KST)
I bet those rumors = him reading the replies from fans 
(Yoochun) To be able to practice the same choreography with you all..... under the same sky^^ It's so beautiful and touching... Because we have you, who can't fall asleep and whose hearts race from thinking about us every day, I spent the whole day smiling, thank you!!! (12:30am KST)
This is the choreography they are talking about
:
Official Flash Mob Choreography for JYJ Seoul concert in November 2010. Be my girl remix. Please learn before you see the show. JYJ wants you to dance with them and have fun. Always keep the faith. -- carlost177 on twitter
Btw, carlost is one of their American dancers^^