I would like to install some sort of video camera near our front entry
gate, that will wirelessly transmit its images to our various home
computers on command. In other words, we'd use our PC monitors to view
visitors at the gate, when we choose to.

Here's the scenario: We have the normal assortment of PCs installed
around the house. Plus, upstairs in the hallway we've already built a monitor
into the wall, framed like a photo (and connected to a hidden PC. It
continually displays our innumerable family photos, and has no readily
accessible access to keyboard, etc.)

I would love, upon hearing the doorbell,  to be able to somehow "flip
a switch" at each of the respective computers (including the "wall
monitor", which would need some discrete way of activation, since the
computer is hidden in a piece of furniture) and see who's at the door.
I understand there are wired systems available with their own
monitors, but I don't want a separate monitor. Nor do I want to hassle
with retrofitting a bunch of wires with a wired system.

I already have a wireless home network, if that may be of any use.

Bonus question: Is there also a way, through the computer (or our home
telephone, for that matter) to activate an automatic door switch that
would unlock the gate on command? I'm capable of running low
voltagepower to the gate. But again, I don't want to have to run wires
to a bunch of activation switches around the house--hence my desire to
use our telephone or wireless switches to open the gate.)

Thanks in advance!

You should visit www.x10.com - everything you asked about could be
done with some combination of their wireless camera, PC interfaces,
and X10 controllers.

I work for a company who can do all of this. See our site
www.sesys.co.uk. We can provide a customised solution or something off
the shelf.

Yes, it can piggy back on to your home wireless network.

The camera operates using Power Over Ethernet (POE) all you would need
is to run a network lead or have a method of supplying mains, maybe a
UPS.

An RS232 port on the camera would allow you to operate a switch
remotely, for gate access by example. You could use the same power
source to do this.

The camera can alert you if something/someone is present, you would
only have to listen or look for the alert. If you're out the camera
has its own memory to store this information, so you can watch the
kids whose frisbee is in your garden try and get it back. The main
difference with what we have is that it is all digital and not
analogue. This will simplify your life and make the installation a lot
easier.

To assist further with your bonus question. The camera can text, email
or telephone (cell phones where and if applicable) you when an event
has taken place. Call the camera back using VoIP technology and warn
the kids "Get out of my garden!"

I hope this helps.