Using a Bluetooth or USB remote to advance Keynote slides? What would it take technically for the iPad to be able to allow Keynote slides to be advanced by a remote (like this -  http://www.targus.com/us/productdetail.aspx?regionId=7&amp;sku=AMP18US)?

Or how about via Bluetooth?
The 2Screens iPad app works with their iPhone remote app for Keynote, PowerPoint, and PDFs through wifi. Apple's Keynote Remote app on an iPhone also offers remote control through wifi.