I want to develop an Android app for home security surveillance that will monitor a door. I want to connect a switch reed to the USB port of an android smartphone.
It's possible to monitor this hardware switch connected to the smartphone USB port? The app will send a SMS when the contact is open.
Yes, if the phone has USB host mode and you connect the reed switch to an external USB enabled microcontroller or I/O chip, but unless you have one of the few devices which allows charging while operating as a USB host, your battery will quickly be depleted. (If you have feel like looking in the kernel logs, you could probably just use your switch to apply the USB sense resistor and detect that - no micro needed).
Consider using the headset jack instead, and having the reed switch drive a simple audio oscillator and detect this with the audio APIs. This should work with all phones, even older ones, and you can keep the phone powered through the USB jack.
Another option would be to use the ADK mechanism, in which a more capable external microcontroller functions as the USB host to host (and also power) the phone. A wider range of Android devices support this (or using a firmware which instead implements an ADB host, virtually all devices support it).