The Compiz modal dialogs finally landed in Ubuntu 11.10 Oneiric Ocelot today. There was some initial work for this, but the actual modal dialogs Compiz plugin has just been uploaded to Oneiric.
Here's a video demoing the new Compiz modal dialogs (Unity 3D):
"Modal dialogs" can be turned off since it's a Compiz plugin. You can also tweak the alpha transparency and fade time:
An initial modal dialogs implementation was also started for Unity 2D and is available for a few days now, but they don't work like actual modal dialogs yet (they are not attached to the parent window and also, the parent window isn't dimmed):
The update should be available for Oneiric users in a few hours, but according to the Compiz changelog, you'll have to reset the profile to get modal dialogs because this doesn't support upgrading.
Note: The modal dialogs are work in progress so a lot may change until the final Ubuntu 11.10 release!