I've now recieved a reply from one of the Senior Technical Advisors at AppleCare who was kind enough to follow up with engineering.
Apparently this change was intentional in 10.7.2 to bring the behaviour of the headphone jack in line with the experience of users who have external displays connected via Thunderbolt !
See this support note; http://support.apple.com/kb/TS4113
I've replied to this advice by pointing out that this change actually removes functionality that was present in 10.7.0 / 10.7.1 and requested that the issue is esculated back to engineering for further comment..
I'll post again when I recieve the next reply.
Actually, I have Apple Thunderbolt display with Mac Mini. And I share the same issue with you. I cannot switch to the display speakers when the haedphones are plugged in the socket. As the Thunderbolt display does not have audio output, I cannot see any meaning in this functionality. Now, whenever I want to swtich from my headphones to the display audio, I have to unplug the headphones from the socket. It would be much easier to switch via the software. I do not understand the logic behind this. Do you have any more info from the Apple engineering ?
I have found a workaround using the "Audio MIDI setup" utility (standard tool may be found under Launchpad -> Utility). This allows you to define a new device like "Display Audio working" which does almost the same as "Display Audio" would do if it would be possible to switch to it. Unfortunately, the sound volume bar then does not behave as expected - it does not allow you to change the volume, so it is not a full solution. But it works and you do not have to unplug the headphones from the socket then. If you would be interested in some more details, let me know.
Thank you m1r3k! That works! I;ve been chasing this all afternoon (thanks for the new feature, apple).
Duarte, once you open theaudio midi setup Utility (in finder click the go contextual menu and select utilities), At the bottm left of the dialog box, there is a "+" sign. click that and "Create Multi-output Device" It will create a new audio device and you can tell it to use the HDMI output. It is then available in Sound Preferences and you can select it even with the analog jack plugged in. Since most people will be feeding HDMI into a device that has a volume control (HDTV, Reciever, etc.) this is a pretty great work around.