some roundabout guidelines:
Please make sure they are counter-clockwise.
In Potlatch there's an icon that shows its direction.
No need to add oneway=yes.
The roundabout should have its own name, it should not have the name of a crossing way.
If the roundabout doesn't have a name, leave it nameless.
The classification of the roundabout way should be like the highest of the crossing ways.
e.g. if a "primary" and "tertiary" meet in a roundabout, then the roundabout should be "primary"
See a good example here:
This roundabout has its own name, doesn't specify oneway=yes, and is a tertiary (and not residential)
See the wiki here:
http://wiki.openstreetmap.org/wiki/Tag: … roundabout
Try this for checking bad roundablouts
Data from cloudmade 05.05.2009