For those who are interested, Wonder festival will be happening Febuary 6 in Makuhari Messe (same building as TGS) and cosplayers are welcome to attend. The official site is here: http://jin.jcic.or.jp/en/anime/animenews/2010_11_25/
Just as a brief synopsis of the event though its meant as a showcase and selling market for Garage Kits (the nice sculpted anime models/figures)
As far as I know its only going for one day....
If I find out any more news I will post it here. I you are planning to go as well then please post =)