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Depends on how it is setup. You want to avoid using cgi parameters in your urls (use standard html/htm file extensions). Other than that, Postnuke works good and attains good rankings if setup properly to mask those cgi urls.
I would add to Brett's advice to look carefully into what urls you don't want spidered as well. With a lot of dynamically generated pages for membership etc spidering can eat up a lot of processor time, data transfer etc.