RemotePad Server is the server app for RemotePad for iOS. This suite of apps allows you to use your iOS device as a remote trackpad to control your Mac OS X computer. There are server apps for Mac OS X and Windows, and you can compile versions for Linux as well. This is an open-source project. Thus, you can download both the server app and its iOS counterpart for free.
Setting up and using this solution was a breeze for me. I first downloaded the iPhone app and then installed the server on my Macbook. I then launched the server and the client, and I could easily connect them both. After a few seconds I was controlling my Mac with my iPhone. On the iPhone, you will see a black space that you can use to control the pointer and three buttons: left, center and right, which control the mouse buttons. They all worked like expected.
I noticed that the mouse pointer moved very little with every full swipe motion I did. This is probably configured this way to let you be more precise with this remote trackpad. I was used to a more responsive and sensitive input, but I guess it is just a matter of getting used to this app. Perhaps a configuration window would be useful for those who want to adjust this setting.