<p>In Google groups and some other web sites, there is a 5-star rating component which is pretty neat, such as in this url:
<a href="http://groups.google.com/group/Google-Picasa-Data-API/browse_thread/thread/b5a346e6429a70a7?hl=en" rel="noreferrer">http://groups.google.com/group/Google-Picasa-Data-API/browse_thread/thread/b5a346e6429a70a7?hl=en</a></p>

<p>I am wondering whether there is an existing 5-star rating component in the iPhone environment. Or if there is not, if anyone has clue where to start?</p>

<p>Thanks</p>

I don't know of any free/available components off the top of my head, but if nobody offers one up and/or you can't find one you like, it should be fairly trivial to roll your own. In the simplest terms, you would only have to:
Create/Find a star icon - one outline (unselected), one filled
Create a UIButton and set the icons for the selected/unselected states
Create a container view that held your x stars and when one was selected, the container could set the selected state on any button to the left of the touched button.
Shouldn't take long at all to build...