I've heard many complaints about the quality of my wallpapers over the time. The main problem seems to be banding (some called it 'compression artefacts' and other things, which really isn't the case!). I experimented a lot and searched many sites about this issue and found out two interesting things:

1.) The first one doesn't really has something to do with banding but is interesting anyway. Windows Vista doesn't list PNG files in the 'Desktop Background'-browser. But you can still make a PNG file your background by opening the contextmenu of the file and choosing 'Set as Desktop Background'. And now the funny thing: Vista converts this PNG file (in my case it was a high-quality 16bit PNG) into a low-quality JPG file. I guess it's impossible to get a good looking desktop in Vista. Can anybody check this out with XP?
It's these little things that make me LOVE vista that much. I hope you get the irony in this one

EDIT: I just found out that there IS a way to obtain the high quality of a PNG. If I open the file with my default Image Viewer XnView (highly recommended, it's free: XnView.com) and choose the 'Set as Desktop Background' option from XnView, it converts the PNG into a BMP. Not a perfect solution but the quality is a lot better than those JPGs.

2.) I read some opinions on the gradient tool of Photoshop. Some folks say, that gradients in Photoshop always have a maximum of 256 steps for each color channel even in 16bit mode. Since I use gradients a lot in my AERO wallpapers, this is a real problem for me. There is one solution: use noise. But I think it's paradoxical to use noise to enhance image quality. And in most cases the noise is obvious. I haven't found any solution for this problem yet, maybe some of you guys can help.