I'm still struggling with this one. If you know of a reliable way to project or use an external monitor from Windows Vista Beta 2 with Aero Glass then please hit the comment button.
I realise that most of you probably haven't had time / access to the software yet to get any where near being able to try this for yourselves.
When I do find out - whether from within MSFT or elsewhere I'll be sure to share it.
The nearest I get @ the moment is to boot the machine with the external display attached - this often works but not always. Otherwise I can roll back my video driver to non-WDDM though that looses the Aero Glass effect. Finally I can often extend my desktop to the other monitor.
Of course another option is to Remote Desktop into the Windows Vista machine though then of course you don't get Aero Glass either.
Windows Vista Aero mirroring support depends largely on the video chipset and driver you are using. For instance, the driver used in my HP NC8000 (ATI Mobility Radeon 9600) and Lenovo ThinkPad T60 (ATI Mobility Radeon X1400) have improved dramatically in the 538x builds. One trick you can try is to boot the machine with the lid down and after you see the login prompt (assuming domain joined or password protected), open the lid on the laptop. This usually kicks the mirroring in and after you login, you most likely see the display settings with mirroring checked and asking for your confirmation. Lately, I have not had to resort to trickery. Usually my laptop just works or the FN+F4 (FN+F7 on the T60) key sequence does the job. Good luck
I have an IBM T41p with an onboard 128Mb ATI Mobility Fire GL video card and an external monitor works each time. The laptop has a function key that is used to switch between onboard display, external display and both.
Similar to Keith, I've found that by having one display offline at boot I can get into Vista then enable the second.
I do it the other way around though - start the laptop with the external display unplugged, and once at the desktop I connect it and switch the display mode to extended.
Steve - is your graphics in your laptop 128Mb? afaik we were told that it was 128Mb per display or thereabouts, I only have these problems with my notebook, the test PC I use (with a 256Mb graphics adapter, also ATI) boots into dual screen without problems whereas the notebook if started with a second display just gives a black screen with flashing white mouse cursor...!