The City of Chicago is on Github
Merging user contributions comes with a number of problems. It'll be interesting to know how they'll manage it. Or, they won't, and this is entirely token. For example, if the city tenders for engineering works, they will likely be required to supply the consultant with source data like road centrelines and road reserves, as-built water networks, etc. That has to be the official, verified, accurate data. User supplied just won't cut it.I'm also dubious that GitHub is the right way to release data. There are a huge number of people interested in civic data, and GitHub is probably one of the least accessible ways for 99.9% of people to get it.For example, a number of city and regional councils in New Zealand publish their data via Koordinates.com. Wellington City Council alone publishes over 50 key datasets:http://koordinates.com/maps/wcc/layers/(disclosure: I work for Koordinates)