Recently found a need to display a caps lock warning on a password field an an enhancement to one of the sites I work on.
I ran across several sites mentioning how to do this - found this one the most helpful at 17thdegree.
Here is the blog post about a rewrite of an existing caps lock notification function using the Ext JS Framework - give it a read if you use that JS framework in your sites already. I had to download and place on our servers, but the implementation looks great and worked great for my purposes.