As stated above, that usually means a bad path. Did you move the folder? (In Leopard, that's not possible. It copies the folder.) If the icon is of no use to you, drag it out of the Dock, and it will disappear. Then follow the instructions above as to the correct side to drag the icon to.right now all i have is a question mark thing there.
Do you mean the Applications folder? If so, that has to go to the right side of the Dock. Read the posts above.I've tried it with the application and it doesn't work,
I've tried it with the application and it doesn't work, I've receive my MBP without the application icon on the dock.
I'm was on Leopard and upgrade to SnowLeopard and still the same, And about dragging, I can drag programs but no folder, so I created a folder shortcut to compensate.
If I tried to drag the application folder to copy it in the dock, it refuse and fall on the desktop, so I've got to drag it back to his original folder.