Unity launcher positioned incorrectly after external monitor detach

1. Plug in an external monitor larger than the built-in laptop monitor.
2. Align the monitors side-by-side with their bottoms aligned
    (in my case I had a 1280x800 and a 1280x1024 monitors).
3. Unplug the external monitor.
4. Press <Super> to make the Unity launcher visible. (It's called the launcher, right? I'm talking about the dock.)

Expected behaviour: launcher is positioned just below the top panel.

Actual behaviour: launcher is positioned half-way down the screen (see attached screenshot).

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: unity 4.24.0-0ubuntu2.1
ProcVersionSignature: Ubuntu 3.0.0-15.24-generic 3.0.13
Uname: Linux 3.0.0-15-generic i686
ApportVersion: 1.23-0ubuntu4
Architecture: i386
CompizPlugins: [core,bailer,detection,composite,opengl,compiztoolbox,decor,regex,place,mousepoll,move,resize,wall,resizeinfo,obs,session,imgpng,grid,snap,vpswitch,gnomecompat,animation,unitymtgrabhandles,workarounds,expo,fade,ezoom,scale,scaleaddon,unityshell]
Date: Fri Dec 23 00:37:41 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release i386 (20091028.5)
ProcEnviron:
 LANGUAGE=lt_LT:lt:en_GB:en
 LC_CTYPE=lt_LT.UTF-8
 PATH=(custom, user)
 LANG=lt_LT.UTF-8
 SHELL=/bin/bash
SourcePackage: unity
UpgradeStatus: Upgraded to oneiric on 2011-10-13 (70 days ago)