At the moment, it is not possible for viewers of your website to leave comments unless you add them as co-collaborators.

I did a little searching and a found a very helpful link that will answer your question in more detail. Look for the comment by "Sites Help Owner".

http://groups.google.com/group/sites-help-howtoend/browse_thread/thread/c3f8b9814d96fc76/50a826fd3867aeb2?#50a826fd3867aeb2

kool