Look like this: Powdery Mildew
If so, a search for it here will pull up several threads with suggested treatments for you.

Yes, they are still safe to eat. ;)

Dave

Or are the "white" patches on your zucchini the normal silver coloration of some zucchs?

A photo would help us give you specifics.

I have posted a picture on the Gallery:
http://forums2.gardenweb.com/forums/load/cornucop/gal09225826896.html

Looking at the photo, I noticed that the stems are also white. Does it look like Powdery Mildew?

Here is a link that might be useful: Photo of the white zucchini leave

Yes, it surely is. But understand that powdery mildew is very common late season. Perhaps it's time to quit?

Generally happens when the nights turn cool. Daconil will prevent it but not cure it. Sign that the season is ending, alas!

Mine are still producing well despite some mildew. As others say, it is common at this time of year and not worth fighting.

Some leaves on a lilac tree nearby also get some white stuffs on it, so I pulled out the zucchini plant already. I also sprayed the lilac tree leaves with baking soda solution.