I just stumbled across your nifty plugin and while my poor Kindle doesn't do all I would like it to in this regard (I also would love it to either let me select the screensaver on device or automatically select the "right" cover for the book I last read), this takes it one more step closer.
In answer to a question I didn't see anyone answer, I personally have almost 300 books on my Kindle, can certainly imagine going into the low thousands, and would love to be able to have covers of all the books on the Kindle as screensavers (as per your quick count, I often power off my Kindle MANY times a day, since I read in short bursts when I can, so I cycle through the screensavers faster). I understand your limit of 99 for avoiding accidental mistakes in adding way too many covers, but here's an idea... make that a max of 99 PER synch. Then I could add all my covers 99 at a time (no clearing).
Secondarily, let me also raise my hand in request of an automatic "add cover when adding new book" option. Of course, for that to work, I'd have to have the higher limit...
Forgot to mention: I'm abundantly willing to Beta test on my jailbroken Kindle 3, though I can't guarantee any promptness as I'm a stay-at-home mom and sometimes my schedule is very free and sometimes out of the blue it gets ridiculous!