If you mean you want to highlight PDFs in iBooks, then no, there is no way to do so. You'll will have to use another app, as others have suggested (I've used Goodreader and iAnnotate with great success).
However, if you are talking an]bout highlighting a book - then yes, you can highlight. See this thread for details on how to do that:
http://www.ipadforums.net/ipad-help/72008-ibooks-highlite.html
Hope that helps.
Marilyn
P.S. @blessedLatina: I deleted your other thread you started today on this same topic. Making more than one post not only fragments the conversation and makes it hard to follow - it's against the forum rules. Please keep any follow-ups, discussion or updates on this topic to this thread; do not start another. Thanks for your understanding.
@all: Please note I've merged responses from the other threads. So, that's why you're here - in case you were wondering what was happening to your posts...