I'm having the same issue. vSphere 5.1.
My customization specification works with one all my Windows 7 templates except one. One template always deploys disconnected from the network.
VMware tools is up to date.
A normal deployment takes about 20 minutes, I let this one sit over night and it was still off the network.
The template is set to connected at power on.
If I convert the template to a VM, it's connected.
We're not using distributed switches.
We currently have 30 VMs on a 120 port virtual switch so we're not out of ports.
I'm stumped at this point.