augmented reality framework

For ideas, you could look at the following:

Existing Apps and their APIs: There are number of Augmented Reality applications in the Android market of which Layar and Wikitude are well known. Others like SomaView and GeoVector also exist. AFAIK, Wikitude and Layar have made their API's public.

Toolkit: There is also a toolkit named, NyARToolkit that provides a library for such a thing.

Tutorial: DevX Article by Chris Haseman

Good Luck!

http://layar.com/ would be the most popular thing I've seen. I made an app that allows for downloading and displaying of information much the same way as layar. It's pretty straight forward.

Hey, I maintain the AndroidARKit, an open source project on github. It's geographic based and includes a simple foursquare example application. More info here. or here. I wrote the devX article linked above. Feel free to contact me if you have any questions. It's licensed for commercial use with attribution. So feel free to use it in commercial apps.

I know that artoolkit has a port to Symbian. Maybe there exists a port to Android as well

The Wikitude API is another alternative for Android applications: http://www.wikitude.org/developers

As reinier said, artoolkit could be the best option. there is for android, check this out