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."

ask_for_finder_items

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."

add_spotlight_comments

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.