Well... consider: this very forum thread is dynamic, isn't it? I have just made a post, and the content of this page has changed. The content is stored in a database, accessed in the PHP script that is vBulletin, through SQL.
Now, search the Web for "Will using a lot of SQL harm my search engine ranking". Has Google indexed this thread?
EDIT:
That said, this may be an unfair example in that the forum is very popular... but the point is that search engine robots are able to read and index such dynamic content, because ultimately it is accessible to them.