I needed to add the same Spotlight
comment to many files so I created a very simple
Automator application to do this.
Launch Automator and drag "Ask for Finder Items" from
the action column to the Workflow pane. Put a check in
the box in front of "Allow Multiple Selection."

Drag "Add Spotlight Comments to Finder Items" (Mac OS X 10.4) or "Set Spotlight Comments for Finder Items" (Mac OS X 10.5) to the Workflow pane, under "Ask for Finder Items". Put a check in the box in front of "Append to existing comments." Click the arrow next to the Options button (Mac OS X 10.4) or click the Options button (Mac OS X 10.5) and put a check in the box in front of "Show Action When Run."

Save your new Automator workflow as an application. Next, drag the application to your Dock. Just click on the application's icon in your Dock to start adding Spotlight comments to your files.