I had the same question last year after buying a house that I needed to monitor while away on business. In the end I settled on a couple of internet based web cams and a couple of ioBridge modules. Now my front door emails me each time it opens! I can also see real-time temperatures/humidity and turn on extra heaters before I get home. I can switch lights on/off and make the place look occupied. and because I know that non of this would actually stop a thief, I also have the camera setup for motion sensitive video capture which ftps onto a remote server, so at least I might have some evidence of the crime.

I hope not, it's still under development! http://whitehexagon.com/