Have you accidently locked your homescreen? It disables changes. Press the menu button and see if "Unlock Desktop" appears, if it does press it.
It's an interesting feature, it allows you to "lock" your desktop to prevent any accidental changes. I think Anderweb added the feature after he made the App Drawer icon editable, and too many people accidently did actually remove their app drawer icon.
Not only that but he turned it into a shortcut rather than the other. However, with the swipe-up command that becomes a moot point -- except for hard-noses like me who insist on having it in the dock.
So, then you long-press, create shortcut, custom shortcut, and open app drawer. Now you take that and replace the dock icon... or just leave one on every home screen.