We have an environment where we have blasted a prepped Windows image out to multiple machines. We now need to customize the networking settings on each image en masse.

What tools exist to modify that image and customize the IP address/netmask/gateway post-imaging?

I would also look at a commercial solution that is able to deploy and customize Windows images as long as it runs on Linux and is fully scriptable.

To clarify, my order of operations is:

* Server PXE boots
* Various Things are done to the hardware to prep it
* Image is copied onto the hard drive
* Image is customized (network settings)

(Naturally, if such a deployment tool exists that can do the last two in one step, I'll take that)
['https://serverfault.com/questions/107330', 'https://serverfault.com', 'https://serverfault.com/users/2101/']
DHCP is the only choice (IMHO) when you get over 5 or 6 machines ( and even then ... ) 

In your case I would also setup dDNS so that your hosts register in DNS when they get a DHCP lease, so if you know the host name you know the ip address. 

As far as host needed a static IP address. The best way is to let them configure off DHCP, but in the DHCP configuration reserve the IP address for that machine's MAC address. This lets you not have to manually configure IPs but still have "static" ip addressing.
If you're talking about servers you **should not** be using DHCP to configure their IP addresses. This creates all sorts of headaches long-term, and even Microsoft will tell you it's not a recommended practice. 

I relate the parable of the DHCP network that got moved to CoLocation and left the DHCP server behind: Needless to say nothing worked at the CoLo and the admin in question looked looked like an idiot.

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For Desktops and Laptops by all means use DHCP, possibly in combination with static leases if you need to ensure that machines always have the same IP when they come onto your network. This is the most sensible configuration for your end users as it lets them put their laptops on other networks (home, hotel, starbucks) without jumping through a million hoops.