You can make any app "invisible" (or at least any Cocoa app) with a little plist editing. Here's how:
1. Find the application you want to change in the Finder, control-click on it, and select "Show Package Contents".
2. Open the "Info.plist" file in the "Contents" folder in any text editor.
3. Insert the following lines after the first "<dict>" (which should be on line 4):
Code:
<key>LSUIElement</key>
<string>1</string>
4. Save changes. If it says you don't have permissions, save the file to the desktop, and then replace the original in the Finder, entering your admin password when prompted.
5. Briefly move the application. This is necessary to make the OS notice the change. If it's in the Applications folder, move it to the Desktop and then move it right back.
There. Now when you load the app, it will have no Dock icon. Keep in mind that this can make regular apps very difficult to activate and control. For example, you wouldn't be able to access the app's menu bar.