I too have this question.  As far as GIS goes, here's some ideas:

http://forum.processing.org/topic/creating-contour-maps w/ some tips on using geotagged tiffs

http://wiki.enterpriselab.ch/el/doku.php?id=projects:colladaloader:colladaloader  - collada will load kmz files made with google sketch up; I am not sure if it imports any georeference information

There is no Processing API for working with Google Earth or OpenStreet maps that I'm aware of.

http://www.gicentre.org/utils/ - This is a processing library with some GIS support/functions. 

There may be more support using Java libraries (which you can do from within processing IDE).  See, E.G., http://code.google.com/p/geo-maps/.  GeoTools might work within processing; http://www.geotools.org/

Google java gis.  If you just want to display maps, then take a look at http://openmap.bbn.com/.  It's for javabeans.  Maybe it works from within Processing.

Not quite the same thing but  Joey Skully has a neat little example using off-line datasets. Plus there's an interesting Weather Radar project over at ruby-processing heading in the same direction.

Also take a look at...

http://www.tom-carden.co.uk/2008/02/18/modest-maps-vs-processing/

It's for Processing and very easy to use.