Once the plugin is installed, you'll notice a discreet slideshow icon in the lower left corner when you mouse over a slideshow-appropriate thumbnail on one of the abovementioned sites. Click that icon and watch your monitor fill with the slideshow. Then you can navigate by using the onscreen slideshow controls ala iPhoto or use your keyboard - or even your Apple Remote! To exit out of the slideshow and return to your browser window, just hit your escape key. If you're like me, you might not be impressed with this concept at all until you've tried it for yourself - but trust me, it's pretty cool.
Even more impressive is that for now at least - it's only compatible with Safari, which is a trend I wouldn't mind continuing! You'll need 10.4 or later as well since it needs Safari 2.0.
You can download Beta 1 for free and if for some reason you don't like it or have a problem with it, you'll find the "Remove PicLens" menu under the View menu in Safari. Let us know what you think!