Step One - Starting with a single address field
Edit Column > Add Column by Fetching URLs
Nominatim has a limit of 1 geocode per second so make sure to set the throttle delay to greater than 1000 milliseconds
Fetch URL based on column (quotes needed):

"http://nominatim.openstreetmap.org/search?format=json&email=[YOUR_EMAIL_HERE].com&app=google-refine&q=" + escape(value, 'url')
 
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Step Two - Extract lat/lon from newly created JSON blobs
Edit Column > Add Column based on This Column
Parse Json and concatenate latitude, longitude:

with(value.parseJson()[0], pair, pair.lat + ',' + pair.lon)

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Sources: 
https://github.com/OpenRefine/OpenRefine/wiki/Geocoding
http://wiki.openstreetmap.org/wiki/Nominatim
https://gist.github.com/mjrich/3118660
http://www.youtube.com/watch?v=5tsyz3ibYzk&feature=player_embedded