For whatever reason, my Xen install wasn't bridging between eth0, xenbr0, and vifx.x. The nic is set up as eth0, xenbr0 is the bridging interface, and the vifx.x are the virtual interfaces for the domU. To fix this, I did the following. Keep in mind, when you do this, you'll lose network connectivity for a moment, so you'll want to add this to startup somewhere. I'm sure I'm missing some config var that makes this work, but I wanted to post this before I forgot the fix.
brctl addif xenbr0 eth0 dhcpcd xenbr0