Thanks to bruno I found this nice entry on jluster.org where Jonas Luster writes about his experiences while searching for articles about this years SXSW and about the problem with multiple tags (for example on technorati) associated with one topic. He also names the lack of tag-aliases as a big problem here.
I also had this problem while publishing entries about my TechnoratiTags plugin for Nucleus. Since the Nucleus website is located on http://www.nucleuscms.org I thought it would be good, to tag the plugins with “nucleuscms”. After checking technorati I noticed, that there are far more entries with only “nucleus” as tag… In this case a small alias would be really useful. Or (as Jonas described it) some kind of “official tag” for a certain topic.
In a comment Buzz Andersen also points out, that there seems to be a feature on del.icio.us in development to bundle certain tags to a category as can be seen here: http://del.icio.us/joshua While this might look nice, I’m not so sure, if this wouldn’t be against the whole tagging concept.