I've seen this appear a few times too. It must be a Leopard thing.. You can delete it. But actually, it isn't doing any harm being there. So it can be left with no ill effects. Just select your regular choice of input instead.
However, if you definitely want rid of it.. Then go to your Utilities folder and launch the Audio Midi Setup app. Go to the pull down "Audio" menu and select "Open Aggregate Device Editor"
From here its quite clear what to do.. Highlight "Built-in input (aggregated)" and hit the delete (-) button and click Done.