Update : For iPhone 4 only, new guide.

Here is how to easily modify your "Folders" icon and his background, and also the backgrond in the multitask bar with iOS 4.

Of course, your iDevice must be jailbroken with iOS 4 and you must know how to SSH your iDevice.

• jailbroken iDevice.
• The icon of "Folders" is FolderIconBG.png
• The background of "Folders" is FolderSwitcherBG
• Without any applied theme in Winterboard, place these 2 files with SSH in System/Library/CoreServices/SpringBoard.app
• With an applied theme in WinterBoard, place these 2 files with SSH in Folders/Springboard.app or /bundles/com.apple.springboard of your theme.
• The background must be a standard wallpaper 320×480.
• Pack with icons and wallpapers

iPhone 4

• Icon of Folders : 118×124, name : FolderIconBG@2x.png
• Size of the wallpaper : 640×960, name : FolderSwitcherBG@2x.png
• With a WinterBoard theme, place these 2 files with SSH in Folders/Springboard.app or /bundles/com.apple.springboard of your theme.