This plugin hides the Winamp agent icon when Winamp runs. So if you have the agent enabled, and tell Winamp to show up in the systray, you will only have one systray icon.
Note that this plugin overrides the settings for Winamp agent showing in the systray.
To install, copy into your Plugins directory.

Download plugin (version 0.2) (20kB)
Download source (3kB)

Thread in winamp forums