Over at Incredibleblogs, there's an in depth investigation of the metroblogging phenomenon in a series of three posts:

1. Metroblogging
2. LA Observed
   
3. Gothamist

Blogging about the same city works for a group blog endeavour because everyone brings their own interests and perspective about their city, & give us as readers a diverse and interesting picture of the places we're "visiting". The blog, like the city, contains an experience and a thread for everyone.

See the Austin Bloggers Meta-blog for another example that gathers Austin TX-related posts from their original blogs & republishes them on a shared page. As I have previously suggested, experience the cities without buying a 'plane ticket. Excellent.