Twitter is testing a geolocation system that automatically detects and marks where users are Tweeting from. The feature is opt-in, so if you're concerned about privacy then you can turn it off.
Interestingly, however, it'll be available to third party developers - so if you're a regular tweeter then it should be relatively easy for someone to knock together a mashup that tracks your movements on a map.
Some users have reported that red pin-shaped dots are appearing on a map alongside their tweets - though many are also placed off the coast of Ghana at 00 latitude and 00 longitude - this is likely to be a result of the service being unable to detect their location.
A Twitter staff member, Raffi Krikorian, announced the development by saying: "As some of you may have already noticed, we've started going through the first steps to get the geolocation API out our door. There are a few more steps in the process that i want to share with all of you".
"There will also always be a tag in the status object regardless of whether there is a location attached to the tweet or not. If there is no location, then the tag will be empty. If there is a location, then the tag will be populated".
The new feature isn't far off a full release, Krikorian says: "We've deployed for internal testing, and soon we'll be turning this on for the general audience".