Yes, you can buy either the composite or component cables from Apple ( Radio Shack stores and I'm sure other iPod sellers, carry them too). They're $50 each, but they include a small USB wall power supply. They plug into the bottom dock connector.
They supposedly work for both the iPhone and Touch. Of course, they're not RGB output to a projector, so if you can't use one of the above types, you'll need a converter. (Many projectors also take composite, but check.)
The downside is, Apple only allows video output for the video and photo apps. So no viewing of the other apps or homepage etc, like Jobs gets to do.
I bought one for my Touch so I could show family vids/photos on our big screen TV. On the upside, people can't see what icons you're clicking, so the show to them is seamless.
If you're trying to show a power point, convert it to individual pictures or a movie.