I'm curious where, geographicaly speaking, are all those YC companies are now.

I can see some main options:

1) They stay where they joined YC. Either they remain in Cambridge or Mountain View.

2) They go home. They return to where they were based before YC, possibly because of visa issues or prior commitments.

3) They move to the other YC hub. For example justin.tv moving coasts after kiko.

Are there any rough numbers available?

Here are a couple: http://www.flagr.com/maps/40

Quite a few of them are in San Francisco, too.

A whole bunch of them have moved to San Francisco in the last 6 months.

a starter list can be found at yrumours ~ http://yrumors.blogspot.com/