Red Monkey [App Store link] does something I've always wanted... a one touch button to dial a number. The free app can be placed on the main home screen of the phone and easily configured to dial a favorite number. In addition, it can be set up to start an email, map your current location, or visit a favorite website. You can only configure one task, so pick the one you want and you're good to go. While the app is free, the developers will sell you an additional 4 buttons for $0.99US each, allowing you a total of 5 buttons for 5 different tasks (if you want to check out RM's competitors, just search the store for 'dialer' and you'll find plenty).
One drawback is you have to remember which button does what, because there is no way to use anything but the generic icon that comes on each button, although after you tap it the button tells you what it is doing.
The app works fine on an iPod touch as a customizable launcher, but naturally you will need a WiFi connection for any of its network-related functionality.
I think this sort of feature should have been built into the iPhone by default; and who knows, maybe it will show up someday. I think this little app is worth a try for free, and if you really fall in love with it you can always add more buttons. There is a pretty comprehensive FAQ on how to use Red Monkey at the developer website.