I'd like to add some exclamation marks to that. GIS functionality should definitely be added to Maps and Semantic Maps - don't go off writing a new extension! There are multiple reasons not to do that, which I'll happily sum up if needed.
Jeroen De Dauw
Don't panic. Don't be evil. 50 72 6F 67 72 61 6D 6D 69 6E 67 20 34 20 6C 69 66 65!
Maps/Semantic Maps added this feature really recently (version 0.6.4
> - I'd like to have a go for an SMW extension that reads in KML  or shape
>  files, in order to show polygons on a map.
Support for shape files would be really nice as well though.
Yes, support for shapefiles and other GIS data, plus greater support for KML, would be quite nice, although in my opinion that should all be done via the Maps and Semantic Maps extensions.
On that note, could you explain how your priorities for Semantic MediaWiki development work get decided, now and in the future? Is it entirely up to you what you work on?