Where is my iPhone backup kept? (Created prior iOS5 upgrade)
If you are on WinXP - 

C:\Documents and Settings/&lt;user&gt;\Application Data\Apple Computer\MobileSync\Backup


If you are on WinVista/Win7 - 

C:\Users\&lt;user&gt;\AppData\Roaming\Apple Computer\MobileSync\Backup


If you are on MacOS - 

~/Library/Application Support/MobileSync/Backup/


With the above path, if you still can not find the iPhone backup location, you may try an application called "iPhone Backup Unlocker", it will help you recover lost backup for your iPhone.