Tracy and Matt

Previously available as website followformation.com, now it's available as an iPhone app that is not free however ($1.99) and it has been released merely 2 days ago:  

Followformation is a tool that helps you follow the top people on twitter by city and by interest. Simply login to Twitter, choose categories or cities that you're interested, browse through lists that are generated using our algorithm, and choose to follow each person one by one, or choose to follow them all. It's a great way to find awesome people to follow on Twitter without all the searching. We find them for you - and make it easy for you to follow them. Features: - Follow top users on Twitter in one tap - De-select certain users that you want to omit from your following group - Preview each user in each category and read their previous tweets to get a better idea of who you'll be following

Commentary: it's a bit disappointing to see that this app is not free - particularly because we cannot check what cities are available before buying - but generally it looks like you can use this app if you are interested in following tweets in some particular city or tweets covering some particular interest. Get it here (price: $1.99):

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Apart from iPhone, iPad and iPod touch, you might be interested also in Windows Mobile phones!