socialight is a new location-based service (beta version currently, it actually started out as an academic project at ITP in early 2004):

Socialight is a mobile phone and web based platform that allows users to create and share location-based messages called StickyShadows™. Socialight's mobile and web tools give you access to location-based media on your mobile and on the web.

StickyShadows are virtual multimedia sticky notes that you create using your mobile phone or this web site. A StickyShadow is made up of media, such as text and a picture, and information about who can see it and when and where it's available. The best way to learn about StickyShadows is by creating and using them. Want to to create one now?

An interesting poster about it is available here. The website also provides a good description of what the messages are.