You cannot directly do that using the iPod Touch completely. You can do one of the following:
1. Install a disk-sharing app like Air Sharing or Files and upload the files to the directories under those apps. They support .key files as well as other formats. You can show the presentation to others but it is limited to the small viewing screen.
2. You can export Keynote to movie formats and sync them within iTunes. Then you can see them. As above, you can show the presentation to others but it is limited to the small viewing screen.
3. If you have iWork '09, then install Keynote Remote which allows to control your presentation on the laptop/desktop. However, you need to be on the same Wi-Fi network as it uses Bonjour to advance/retract to slides. This is the closest to what you wanted.
It is possible that the new mobile OS 3.0 will support additional hardware (like the diabetic slideshow) that will allow you to connect external hardware to the Touch. But that will occur after some time unless Apple releases the info to manufacturers. I do remember David Pogue reviewing the Optoma pico projector but it will display only movies, not Web pages, from the iPod Touch. Not sure about presentations.
Edit: Found
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