To stream surveillance video to the iPhone, tap into one of the many available Web cameras (like this one from Axis) that wirelessly stream h.264 video to the Web, and then use Mobile Safari to capture this stream on your iPhone.
For a more sophisticated approach, Macintosh users can try SecuritySpy, software that offers support for multicamera monitoring, motion detection and AppleScript support through a Mac client. The iPhone-specific app that works in conjunction with SecuritySpy, Cam Viewer for SecuritySpy, is US$2.99 from Furnishing Industry Software, which also offers a version, Cam Viewer Lite (also US$2.99), that simply requires an IP-enabled camera.
For those wanting a less sophisticated approach, there is an application called Air Cam Live Video that allows the streaming of video from most webcams to iPhones and iPod Touches. Since Apple ships iSight with its entire portable and iMac product lines, anyone can use the cameras built into these machines with Air Cam Live Video to stream video to iPhones locally or across the Web.
Similar to NumberKey Connect, Air Cam (US$7.99) requires a companion application to be downloaded and run on the host computer, either a Mac running OS X or a PC running Windows XP or Vista.
Once the application on the host computer is configured with a password (which is necessary to log into a video stream), Air Cam automatically displays local connections as well as the ability to manually connect to IP addresses.
Air Cam even comes with simple and straightforward instructions about how to configure access from beyond the local network, and the video stream works remarkably well both over Wi-Fi and 3G access.
This application is well worth the price for anyone interested in what goes on when they're not around.