If you have multiple copies of the Developer tools installed on your Mac (say, one for SDK 2.2.1, and one for 3.0), somebody on Twitter (maybe Craig Hockenberry?) suggested altering the icon of one of the two copies of Xcode so you can tell which is which in the Dock. I thought that was a brilliant idea.
Here is an icon file you can use if you want. Right-click and choose "Download Linked File" to save it.
This is what it looks like (nothing too fancy):
To change the icon, navigate to the Applications subdirectory of the beta /Developer folder, then right-click on Xcode.app and select Show Package Contents to open up the bundle. Once the new window comes up, drag the new appicon.icns to the folder inside the bundle called Resources, choosing to replace the existing file.
You may need to log out and back in to make the change take effect in your dock. You could also quit the Dock from the command line, or remove and re-add the beta Xcode icon.