Can't speak for iWeb, but first do you mean for images or text?
For images (block elements), there is an excellent
cross browser solution here using standard CSS that involves simply adding a class name to your image and uploading a few simple background images. The fact that it's DW doesn't matter, no special plugin, you can manually apply the style or use the GUI to do so.
For text, we're limited in solutions. Avoid the CSS techniques that involve filters
as documented here --- they only work on MSIE. Mootools has an interesting
plugin as shown here that overlays text slightly offset and gray colored, but Mootools is a JavaScript framework and requires a small amount of hand coding to implement. JQuery has a similar plugin and
this demo is very effective, but it's still a framework but from what I can tell alot more advanced than Mootools based on the demo via my FFox.
But what most people do so they control the exact look and feel is use graphic editors such as Adobe Photoshop to add shadow and other blending effects directly to images. Of course you can embed text in images, but only do so for your logo or masthead, do not abuse this because it eats up bandwidth and renders the page much more slowly. So keep the fancy stuff to graphics only, not text content on your site.
Bottom line is you might need more than one technique. I personally would use the first CSS for content images, i.e. photos in a gallery page or something, and use JQuery for text and anything else. But in a fancy masthead or company logo, Adobe all the way, optimize then save for web and it looks the same to everyone. Get the idea?
I'm sure others here will offer additional ideas. Just to get ya started.
-jim