A remote control device is required equipment for the serious presenter — my favorite remote (and I have many) is the Keyspan Presenter. So I was rather skeptical about Apple's Keynote Remote, an iPhone (or iPod Touch) app that turns your iPhone/Touch into a basic remote if you're using Keynote '09. It's ridiculously easy to set up (the devices communicate over Wi-Fi), and...it just works. It won't replace my Keyspan (the Keynote Remote app does not have a black-out-screen function and the iPhone is a bit bulky in the hand compared to a tiny, plastic remote), but if I ever forget my remote control, my iPhone will do the trick just fine. Below is a rough cut of my impressions with the Keynote app today.
Here's a good review of the application on appleiphoneapps.com.