I've always been jealous of Picasa's Slideshows under Windows. They don't work well in Linux' Wine version unfortunately. Until today. I wrote an email to the developer of the great kde screensaver "smooth slide saver" that I've mentioned just the day before yesterday.
And he told me about a digikam plugin with similar features. It was pretty hard to find. Even after looking up screenshots of it being used. It's called digikam advanced slideshow plugin. And it's part of the kipi-plugins package.
You need to install the packages digikam kipi-plugins. Then you start digikam. Don't select the normal boring slideshow that's normally used. Instead select a couple pictures in an album, go to Extras (in the menu), select (Advanced) Slideshow, right underneath Find Duplicates. And enjoy! :)
And the great thing: Digikam and with it the (opengl) advanced slideshow plugin support the comments I put into the pictures with Picasa. That means I can do the perfect slideshows under Linux now, even much better than Picasa'a Windows slideshows, with all my comments. And all open source! Cool.