It might be possible that while jailbreaking iPhone 3G on iOS 4 using redsn0w you have enabled backgrounds to give the icons a nice shadow effect.
The instructions are given below:
1) SSH in as root and install ericas utilities
2) Navigate to /System/Library/CoreServices/SpringBoard.app
3) Convert the plist we need from binary to xml so you can edit it in a text editor:
plutil -convert xml1 N82AP.plist
4) Edit N82AP.plist file, and change this line:
6) Navigate to settings, general, spotlight and uncheck everything except apps and contacts.