I've been running Vista Beta 2 (Build 5384) for a few days now but the Aero Glass wasn't enabled for me.  The Aero Glass gives Windows-Key Tab 3D switching between windows, the hover over the start menu items shows a cool live screenshot of the window, and the regular Alt-Tab is enhanced.  There are other 3D and enhanced visual affects as well.

It worked when running one of the earlier Vista Beta builds and my hardware is new so I figured I shouldn't have a problem figuring out how to turn it on.  Well, it wasn't as straight forward for me so I figured I would post information that I found while trying to enable it on my computer.

The performance rating score that your computer appears to be the determining factor on whether or not Aero Glass is enabled.  If you can get your score high enough, then it is supposed to turn on by itself

http://blogs.technet.com/steve_lamb/archive/2006/05/26/430479.aspx

My computer was only rated at a 2 because of some boot up problems.  I viewed the information provided and the issue for me was that the Vista Search features took a while to load once which dropped by rating.  But even when I cleared the Event Log info, rebooted and refresh the performance score, my rating was always 2.  After some time I figured I would move on and worry about this another day.

You can override this check with the following registry key change: http://www.neowin.net/forum/lofiversion/index.php/t385504.html.  This can be applied to HKEY_LOCAL_MACHINE or HKEY_LOCAL_USER depending if you want it computer wide or user wide.  This seems to be the fix that helps the most people.

But even this didn't work for me, so I kept looking.  This blog of Julie Lerman's mentioned to get the vendor's latest drivers, so I went ahead a tried that: http://blog.ziffdavis.com/devlife/archive/2006/04/09/40828.aspx

No go.  Additionally there is another skin that can be downloaded that apparently achieves the same thing but works with lower end video cards: http://www.neowin.net/forum/lofiversion/index.php/t296493.html.  Some people seemed to have good success with this.  I didn't try this because I was determined to get the native Vista glass skin working, but I almost reverted to this option, and it seems to be a good option for many people.

Here's a potential fix that requires running WinSAT Aurora: http://chris123nt.com/2006/03/25/regain-glass-in-5342.  Again, it didn't work after this fix, but it may be what is required for some.

None of these worked for me, but after some poking around I finally figured out what the problem was.  I had to change the "Appearance Settings" from "Windows Vista Basic" to "Windows Vista Aero".  Very possibly I changed this to the basic appearance while poking around a few days ago, so I'll take the blame for it being wrong.  You can get to this by Right-clicking on the desktop -> Personalize -> Visual Appearance.  Note though that this screen is different, depending on if you have Aero working or not.  If Aero Glass is working, you'll get a screen which lets you change your color scheme.  Otherwise you'll get the classic appearance properties.  In here, I changed the color scheme to Windows Vista Aero and everything worked finally!  (if you get to the "Change your color scheme" screen instead of the Classic screen, you can click "Open classic appearance properties" at the bottom to get back the classic screen, but if you get the enhanced screen, you probably have Aero Glass working)

So, in my case I'm not sure which step or steps would have solved the issue since I didn't have the glass turned on in the first place. But hopefully one of these steps will be useful to you to get the Aero Glass working on your Vista install.