If you've got a jailbroken iPhone (which, btw, is now possible with firmware 2.2), you can easily control it from your Mac (or PC) using VNC.

Being able to control your iPhone using VNC means:

1. Full keyboard support on your iPhone -- type fast, save drafts, save notes, copy-paste from computer to iPhone (in some apps, like Notes)
2. Access iPhone from any computer in any room on your local network
3. Be as lazy as possible; avoid reaching or moving to check text messages
4. Remote control iPhone's iPod when connected to your home stereo
5. I suppose you could port forward and access your phone from anywhere, but then, why wouldn't you be with your phone?

Installation is a breeze; open Cydia, search for and install Veency (thanks to author Jay "Saurik" Freeman), restart Springboard. Go into Settings, Wifi, click the blue arrow next to your network, note your IP address. To connect to your iPhone, you'll need a VNC client like the free Chicken of the VNC; the Mac's Screen Sharing app doesn't work for some reason.

In CVNC, setup a Connection Profile for your iPhone; make sure to map the middle and right click, also set Color to "thousands." Connect to your iPhone's IP address using the Connection Profile you just created (go to File » Open, instead of File » New to use a Connection Profile). Once connected, you click and drag just like using your finger (which is represented by a small cursor). Left click for clicking, middle click to lock screen (you can even drag-to-unlock), right click for home button.

You can set the middle button in Chicken of the VNC -- I use a double click of the command key. You can even "flick" the screen up and down if you time your click-drag-unclick properly, and double click the home button on the lock screen for your iPod control (or whatever you have double-click set to in Settings). If you need more details (including screenshots), you can find them in this blog post.