There are some cool Guinness ones I'd like to make (for personal use because most of the material probably couldn't be repackaged on other sites!) but I can't find any material of high enough quality (i.e. 3200 pixels wide!)
im a bit tied up now, but i did a quick test in liqbase of a couple of things like that a while ago.
i have a function which will just allow the background bitmap to softly drift (that actually works on any picture in the system with a single switch), and I can have a middle layer between the background and the foreground which can be occupied by stars or flow or any other widget i want
ill show it in motion somewhere later, just subscribing to this thread tho
I find the color green very soothing as a background image, so I took a fantasy pic I had on my desktop computer, beefed up the green and released it on n900wallpapers.
It's got three pretty abstract backgrounds that look nice with icons on top and one with a nice woman's face in the left corner for "wow-effect".