August 26th, 2006
The Installer icon could use differently colored arrows to communicate the progress of the installation process.

Multiple simultaneous installations would have to be aggregated.
Icon badges might be better due to color perception differences. Yes, I’m aware of the large percentage of people with color blindness.

As suggested in the comments, the orange pie progress badge would indeed fill as the installation proceeded. The widget is already used in Mail and the forthcoming Safari 3.0 release with 10.5.
Notification methods such as Dock icons and Growl bezels will be all the more useful when virtual desktops spaces are introduced with 10.5.
February 10th, 2006
I took a minute to update the look of the iTunes Dock badge I posted long ago.

The orange for in-progress and green for completed are consistent with the status icons displayed in iTunes when importing from a CD and would make it easier to visually determine the status than when using red for both.
May 17th, 2005
While waiting for the updated and improved MailAppetizer release, I’ve been using GrowlMail, part of Growl, Christopher Forsythe et al.’s open notification framework for Mac OS X. Given the conceptual similarity, it was easy to adapt my MailAppetizer priority coloring idea to the Music Video notification design:
GrowlMail Notifications with Priority Level Indication
The three colors would map to the three priority levels that can be assigned to outgoing messages using Mail 2.0’s built-in functionality (there are actually five X-Priority levels), decreasing in visual prominence as the priority level lowers.
On a somewhat related note (look at the image), Path Finder 4 is going to rock my socks.