Hi,

I am looking for advice on building an A/B type switch to allow the
sharing of a single IDE ATA Hard Drive between two systems.

Use and power supply would be by only one system at a time.  I
intend to fully shut down one system before switching access to the
other machine and then powering it up.  

I was thinking possibly a Y adapter of some sort.  Very easy to find
for the power supply but not sure if its readily available for the ide
cable.

In a perfect world I could also set up an A/B switch to control both
power supply and ide cable connections.

Real-world practical application would be to share out the hard drive
contained within my PVR (basically a TIVO-like device) with my PC so I
can rip the stored content.  I can so this now by removing the hard
drive and hooking up to my pc, but I would like to add some polish to this
process.  I would also like to eliminate the need to open both of my
devices each time I want to do this.

Possibly the solution could involve one ide connection and one usb
connection (the DVR machine would require the ata, but my pc could
accept the usb).

Any ideas are appreciated

The first problem you would have is the 50cm (18 inch) lead length.
The PVR and the PC motherboard would have to me mounted very close...

The second problem is a legal one (copyright issues and the like).

Assuming that the two above problems are overcome (you have the
permission of the copyright holder, and the PVR and the PC share the
same case!) then such a device would be technically possible to make. 
 Basically, it would consist of an AVR or PIC microcontroller
controlling a bunch of gates... the first to see a +5V signal would
get the ATA device.

Mass production would be cheap enough, but the cost of designing and
debugging  the circuitry / writing the mocro code is going to be in
the low 10s of $K.
Not worth it unless you are going to make less than tens of thousands of the units.

Can't you add a removable hard drive unit, so at least its a matter of
unplugging one and plugging into the other?

The most practical solution I can thing of would be a removable
hard-drive caddy (like this:
http://www.mycableshop.com/sku/BAYATA100.htm ) - however this would
rely on your ability to modify your PVR to facilitate the mounting of
the housing bay.

If that is possible for you, it allows for another advantage - it
would be easy to have two hard drives in play - one mounted in your
PC, and one in your PVR. When you wanted to swap them, it is only a
minimum of downtime for both, and nor specific time pressure to copy
data to your PC in order to bring your PVR back online.

Simple Answer: Not possible

Elaborate Answer: I've never seen any type of device that would allow
that, simple because what you're doing to your PVR (removing the hard
drive) isn't an intended function of the unit and my guess is that it
isn't a piece of cake to remove the PVR's hard drive. My best
suggestion would be to buy a simple basic PCI TV tuner card and use
your pc for PVR capabilities, get another hard drive for your computer
to record video on to (as its likely going to be uncompressed when you
record it) or pirate the drive from your existing PVR. Many TV tuner
cards come with the type of software that allows you to schedule
recording times and channels, thus making your pc into a much more
expensive PVR (were you to build it from scratch). That's the best
solution I can provide.

Best of luck with your project!