Let's not give the architects and those with the drawing and drafting skills the full credit for being urban designers. Yes, they have remarkable rendering, visualization, and presentation skills, however, some one has to write the urban design codes, regulations, and guidelines. And it's on this latter point where professionally trained urban planners can in fact impact urban design. But those trained in LA/UD/Arch have an inate knack for creating, drawing, generating computer visualizations, etc and probably end up doing the actual design. But depending on which sector these folks are from, the resulting final plan may or may not be what the designer intended. For instance, if you are an urban designer in the private sector, working for an architectural and planning consulting firm, you would do loads of drawing, interacting with municipal officials & and the developers, and more loads of drawing. But if you work for a municipality, I would imagine your work would be policy driven, writing about design objectives the local community wants, that is, writing the handbook for FAR, height restrictions, facde and signage requirements, etc, etc. You wouldn't actually do the designing, but your work would impact the overal design of the town/city over time. Further, in this role, your consultations to the planning board of commissioners could also affect how designs are modified as they are presented by developers to the commission.
But I don't know if I answered the original question posed. My sense, and this is the pessimistic side of myself, is that "urban design" is a new, fancy way of rethinking "urban planning," you know, to make it sound more glamorous and arty. It seems as if the new urbanists, and I'm talking about Duanyism here, the foaming at the mouth hardcore adherents of NU, believe that by revamping zoning codes with a visual guide and design standards that they are not merely "planners" but more than that, to be "designers." Since these new design guides have renderings instead of paragraphs and paragraphs about non-excluded uses, procedures for seeking zoning variances, etc, etc, ad infinitim, that somehow, the inclusion of pictures makes a planner a designer. ...hmmm... I'm not so sure about that. What do others think?
EDIT: Check out this thread, it is small, and I gave a long reply (and I do hope others respond! I want to know what other Cyburbanites think of urban design), but it does address the nature of urban deisgn, albeit from a British revitalization program POV:
http://www.cyburbia.org/forums/showthread.php?s=&threadid=1197