Ever since Apple killed the Griffin IR remote on the original iPod I've been wanting to consolidate my iPod with a home theater remote. The iPhone seems perfect for this; with an infinitely variable canvas available, you could theoretically do anything with your remote.
The trouble is getting an IR signal to your components. The UnityRemote from Gear4 solves this problem by connecting your iPhone to a small cylindrical IR beamer via Bluetooth. We didn't get much of a chance to try it ourselves, but the demo was mighty impressive. I should note that Gear4 is not new to this game -- they created an RF-based remote for iPods several years ago but it never took off. iOS devices, however, are much better suited to remotes.
While you have the usual raft of buttons and shortcuts (you can add buttons to your favorite stations with station icons) plus the ability to teach the device your oddball remote codes, the really compelling piece in my mind was the ability to use gestures. You can flick your finger up in the gestures screen to increase volume, much like FlickTunes does on your iPhone or iPod touch. The UnityRemote works on iPad as well, which means you can have a really big remote in your living room.
The app is free, but the UnityRemote hardware will set you back $99. Considering Harmony remotes can cost $200 or more, that's still a pretty good deal if you have a lot of components or you're a control freak.