how to record screensaver as a movie

If you are referring to the slideshow screen savers like "Cosmos," simply use the context menu to open the slideSaver package, open the "Resource" folder, and drop-import the collection of JPG images to an iMovie '08 project. Now you can apply your own Ken Burns Effects and export for your particular target use in whatever compression format you desire.

Thanks for the update... Although I am still unsure whether you are referring to the mosaic created by dropping individual photos or the mosaic that evolves into the next photo. Be that as it may, the process is the same. In this case, as David previously mentioned, your best bet is to use on of the "screen capture" applications since it does not fit the scenario described earlier.
CLICK HERE TO SEE A SAMPLE CAPTURE
As you did not provide specifics as to target requirements, I captured my full 1680x1050 display screen and scaled it to 50% for the sample above. Since I am using an original PPC G5 2.0 platform, my frame rate for a display this size was limited to a maximum in 16-17 fps range, so I elected to capture at 15 fps. (You should be able to better on your machine if you want.) In any event, there should be a number of screen capture applications on the market if you search around. Like David, I tend to stick with Snapz ProX from Ambrosia as I have been using it for a number of years and it has served me well over that period.