Folder actions in Leopard are triggered by the application Folder Action Dispatcher, which basically checks for modifications in a folder at a specified interval. Unfortunately, the default time interval in Leopard is 180 seconds, i.e. three minutes, which may be way too long sometimes. However, Folder Action Dispatcher is a scriptable app and it allows user to change the checking interval. Open Script Editor in /Applications/AppleScript, and type the following command on one line:

tell application "Folder Action Dispatcher" to set polling interval to nnn

Replace nnn with the desired polling interval, in seconds. Now click the Run button in the toolbar, and voila!

[robg adds: On the queue site, a comment notes that Script Editor may ask where the app is -- it resides in /System/Library/CoreServices. You can avoid this by specifying the full path in the above command. It's also noted that there's a new AppleScript command to force a folder to be checked: tell application "/System/Library/CoreServices/Folder Actions Dispatcher.app" to tick.]