If you don't mind spending a bit of money and adding a couple of pounds of weight (and a two hour time limit) to your iPad, this video demonstrates that a simple HDMI wireless extender will allow you to transmit audio and video to a remote presentation system.
After watching the clip, we hopped on Amazon to see what it might take to duplicate this hack; we found the HP Wireless TV Connect (the same product that appears to be in use) for US$150, and a similar Brite-View product for $160. Although you can choose a VGA extender over an HDMI one, you will not be able to transmit audio the way this solution does -- and you'll end up spending more.
So how much should you expect to spend, beyond the iPad and receiving screen? The HP transmitter should work fine, although you can scale up to more professional HDMI transmitters for more dough if you need the quality. The Digital AV Adapter is $39, and the battery pack will add another $24 -- then you'll need a case to hold it all together and stronger arms to carry the iPad around.
Still, for mobile presenters and trainers who absolutely have to roam around with iPad in hand, it's an intriguing and (relatively) economical hack.