You can with a 3G + Data Plan iPad 2 / iPhone and above.
Create a Personal Hotspot wifi with a certain name and password.
Have the Apple TV connect to it.
Now take you Apple TV anywhere with you.
To setup an airplay/projector broadcast.
Start the Personal Hotspot wifi.
Connect the Apple TV to any hdmi projector.
Done. You can airplay your keynote or video to the projector.
This will work with any AirPlay enabled app. Using Keynote with AirPlay is no different.
Tap Keynote to launch it on the iPad, and then open the presentation you want to give.
Double tap the Home button on the iPad itself, and the multitasking bar will appear at the bottom of the screen.
Swipe to the right to reveal the media bar, which has the AirPlay button just to the left of the Volume slider.
Tap the AirPlay button and select Apple TV, then tap the Mirroring switch to turn that on. The TV screen should now have your presentation on it, ready to go.
You can also use this method to send the signal from any AirPlay enabled app video-out to an AppleTV.
Enjoy a "walk around, ipad in hand" wireless presentation.
PS. There are also some older VGA projectors, for those you might need an hdmi to vga "active" converter cable to hook the apple tv to it. Haven't tested this, and that cable is around 40-50$.