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'm using PostNuke and ModRewrite for one of my sites and it is happily spidered.
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.
There are several posts here about what the SEs think of .php versus .html. In the short term - I doubt it will hurt your rankings appreciably. When you start passing vars on the URL then yes, you will see the spiders stall. But a simple .php should be fine.