I just had a interesting phone call with a guy from Genève tourism.
He told me that is possible to host 400-500 people in Geneva keeping
the costs relatively low.
He suggested to use Uni Mail for the conference which will be free of charge.
He also told me that all the people attending DebConf13 will have free
access to public transportations.
He can also help to organize food (lunch and dinner).
His name is Enrico Zuffi and can be reached through his secretary at
christen (at) geneve-tourisme (dot)ch.
I hope to meet you soon.