I can share some experience from our mobile freidae event here in Berlin/Germany.
First it started as a simple user meeting at a lounge/bar.
I've had advertised it on Twitter, t.m.o and my blog.
We were about 10 people, mostly developers.
The next time we made it at the c-base (known from the first maemo summit).
That ws the right decision because nearly everyone at the c-base uses a N900.
Today in one week we will have the next mobile freidae (the second time under the mobile freidae name and the fourth maemo event in Berlin by us).
We will have talks about MeeGo, programs and so on.
Even the guys from Qt Berlin are coming to talk about Qt development.

A little info on the side: mobile freidae is a mix betweet German and English.
What mobile means is obvious.
frei is german and means free, as in Linux. Spoken as the English fri in friday
dae is spoken like the English day and shiuld remind you of maemo.

I made the experience that the event will grow with the time.
A lot of amazing people will involve and make sure it will be a success.
We now have the mobile freidae one time a month, every last friday a month.

I would suggest to met at a really cool locaton, it will grow itself.