I have actually registered on the forum to show support for this. I have been using defraggler for about 6 months now and swear by it. I tend to be very meticulous when building a new PC, but always end up forgetting to defrag once I am finished.
My usual install method - install windows, defrag 3 times, install each piece of software followed by a defrag. I have used this ever since working with terminal servers and noticing the vast improvement in speed over time.
A screensaver would just top this off nicely, meaning that I never need to 'remember' to defrag again.
It would be pretty easy to create a screensaver that is separate from defraggler, but the best feature would be a scheduled defrag option (so when the screensaver kicks in after the scheduled event, it runs a defrag, pauses when the screensaver gets turned off and then resumes until defrag is complete)