Yes you can... It's just a .png file you can swap with another one.
They need to be the same size image and are located in the phone.apk drawable-hdpi folder
YES, YOU NEED TO BE ROOTED
The 2 images attached are for 2.1 (white background) and 2.2 (black background)
EDIT - I zipped the 2 .png's up an attached them because the forum for some reason converted them to jpg and they look like crap (white one isn't supposed to have a big ugly black spot on it)
You can see screenshots of them on the phone in my theme thread here
http://forum.xda-developers.com/showthread.php?t=727117