When booting oneiric Desktop ISO, "try or install" dialog doesn't start

Binary package hint: ubiquity

Oneiric Desktop i386/amd64 20110527

Booting from USB Key on real hardware, or directly from the ISO on a VM:
1. The boot process starts normally (blinking dots)
2. Then the Ubuntu default purple background is displayed
3. Then it switch to a plain blue background
4. The Ubiquity dialog "Try or Install Ubuntu" is not displayed and everything stops there

ps shows that ubiquity-dm is running:
3088 ? S 0:01 /usr/bin/python /usr/bin/ubiquity-dm vt7 :0 ubuntu /usr/sbin/ubiquity --greeter --only

If the user selects "Try or Install Ubuntu" directly from the boot menu, the live session starts, and ubiquity works as expected to install Ubuntu.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: ubiquity 2.6.10
ProcVersionSignature: Ubuntu 2.6.39-3.9-generic 2.6.39
Uname: Linux 2.6.39-3-generic i686
Architecture: i386
Date: Fri May 27 13:47:14 2011
LiveMediaBuild: Ubuntu 11.10 "Oneiric Ocelot" - Alpha i386 (20110527)
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)