<p>I am putting together a SVN demonstration using a WinXP Virtual PC image, and am unable to see my host machine from within the VPC guest OS.</p>

<p>This demo will be performed while disconnected from our LAN, so I installed the Microsoft Loopback adapter on my laptop.  In the TCP/IP settings I manually set the IP address to 192.168.3.1 to avoid conflicts with our internal network.  In the VPC image settings I set the Network Type to Microsoft Loopback Adapter and set the IP address to 192.168.3.2.</p>

<p>I cannot ping my host machine from the guest OS and vice versa.  I've followed through several different how-to pages on this subject but still cannot determine why this isn't working.  What am I missing here?</p>

So your host machine's loopback is set to 192.168.3.1 (subnet 255.255.255.0, gateway 192.168.3.1) and your VM's network adapter is set to 192.168.3.2 (subnet 255.255.255.0, gateway 192.168.3.1)?  They should be able to see each other, there's not much else to this setup.
I'd shake things up:
Install the VM additions, maybe the network driver that comes with the guest OS isn't cutting it.
Try a different subnet, maybe 192.168.22.1.
Disable your host machine's other network adapters for a bit, maybe packets are getting routed elsewhere.