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[highway=mini_roundabout][bbox=15.86,47.95,16.88,48.70]
> wget http://open.mapquestapi.com/xapi/api/0.6/way[junction=roundabout][bbox=15.86,47.95,16.88,48.70]
Note: Not all XAPI servers are available all of the time. Check this site if the server is down: OSM Wiki – Xapi.