This appears to be the issue on referata. I don't know why. It works on my wikis. Where is it not working for you? On a referata wiki?
Either way, I contacted Yaron about this to figure out what's different on this site so the issue can be pinpointed.
Same problem on my wiki as described above with missing exponent JS error. Example: http://fahrrad-wiki.org/wiki/Abiko Thanks.
Edit: MW 1.18.1, Maps 1.0.5
Same issue on Fairfaxpedia, service=osm works but service=openlayers just hangs with "Loading map...".
Turns out this is an issue with MediaWikis 1.18.x resource loading system that is breaking OpenLayers. I can't really do anything to fix this in the extension. It works properly again in MediaWiki 1.19 and later.