Roundabouts are handled interestingly in OSM: They can be both nodes or ways and represented as points, lines or polygons.
And this is how they can be downloaded for a specific area:
> wget http://open.mapquestapi.com/xapi/api/0.6/node%5Bhighway=mini_roundabout%5D%5Bbbox=15.86,47.95,16.88,48.70%5D
> wget http://open.mapquestapi.com/xapi/api/0.6/way%5Bjunction=roundabout%5D%5Bbbox=15.86,47.95,16.88,48.70%5D
Note: Not all XAPI servers are available all of the time. Check this site if the server is down: OSM Wiki – Xapi.