Currently in Twitter there is a single location attached to a user's profile.
This is not really well supported or exposed in the web interface, and most people don't know that you can use near:location as a Twitter search parameter, e.g. near:ottawa or near:45.44,-75.62
There are ways you can declare location changes in a particular tweet, e.g. the l: syntax, a Twitter nanoformat, but these are only supported by a few tools, not by Twitter directly.
The applications that I have seen with the most support for location information are smartphone Twitter clients, they typically have an icon to let you set your profile location to the current location (as determined by the phone), and some will let you do "nearby" searches based on your current location.
Providing location information for web apps has been difficult in the past because there was no API to ask the web browser where it was, but now geolocation is supported in Firefox, Safari, in Google Gears, and on various platforms. This is being enabled by a standard W3C Geolocation API.
Now Twitter has announced the rather more logical option of having a location attached to each tweet, rather than globally to your entire account. This is another step in adding more metadata attached to the raw 140 characters of a tweet. This will make possible lots of interesting tools, one obvious one being to display all of the tweets posted at a particular location.
This does of course have privacy implications - and I hope that the system will provide per-tweet opt-in to sharing of location, rather than a global opt-out. We need systems that understand you may want different granularity of sharing depending on your location - a typical example is that you may want to share your exact location when you're at a pub downtown so that friends can meet you, but you may want to share only city-level location or no info at all when you're at home.
There are already examples showing up of embedded location giving away rather more information than someone probably intended, such as a thief whose photo of stolen goods has embedded EXIF-GPS (via @jblyberg).
The official word from Twitter is
We're gearing up to launch a new feature which makes Twitter truly location-aware. A new API will allow developers to add latitude and longitude to any tweet. Folks will need to activate this new feature by choice because it will be off by default and the exact location data won't be stored for an extended period of time. However, if people do opt-in to sharing location on a tweet-by-tweet basis, compelling context will be added to each burst of information.
from Twitter blog - Location, Location, Location - August 20, 2009
Widely reported, e.g. PC World, New York Times, CNet.
via Dean Giustini (@giustini)
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