Time-lapse movies can be a complicated affair, and quite often involve either a lot of special equipment, and/or post-processing skills. For just a buck though, you can use the recently released Timelapser app (link opens in iTunes) to turn your iPhone into a tool that can do this time-bending filming technique using nothing more than the onboard camera.
Depending on what model of phone you have you can use the app to take a picture anywhere from every three seconds to once per half hour. All the while it grabs each frame and stitches it into a movie that's saved on the phone, and that can also be e-mailed to friends.
Of course if you really want to cook with gas, you'll need an iPhone 3GS, which lets you speed up how fast the phone can take shots. Alas, with my lowly 3G I was limited to taking a shot every six seconds. Owners of the original iPhone have to step it down to eight seconds.
The app has a wealth of settings that let you pick things like how large the video's resolution is, how many frames per second it should be, and how long you want the delay to be before it starts shooting. This can be useful if you're propping up your phone somewhere and need time to set up your scene. Users can also use the app just to take a series of photos one after another which get saved in your phone's camera roll.
As I noted when I checked out the IP Camera app, which can, the very best way to use this app is with one of Apple's fancy docks. You can also just prop it up with whatever you may have laying about the house, but with the dock you get the benefit of being able to keep it plugged in. This is incredibly important if you plan on shooting something over a couple of hours. You may also want to turn your phone sideways so that your videos get shot in a typical widescreen style.
Here's the test video I did. My settings were 360 × 480 pixels, taking a shot every six seconds, which came to a grand total of 788 individual photos (thankfully none of which were saved to my film roll). The whole thing took about 10 seconds to process and save to my photo library when the app was done: