I'm running Ubuntu Linux, Dapper Drake on one machine, and Windows XP Pro on another machine. Currently, the Windows machine is routing connections, however very soon I'm getting broadband and an actual router will be doing this. For the time being, is there any simple way to make the Windows machine give the Linux machine a static IP address? Re-configuring my port routing rules every time a machine reboots or does something funny is a royal pain.

You should be able to set the windows machine not to give the Linux machine an IP at all, and then set the IP of the Linux machine manually instead. How are you routing it, though? ICS?

Yeah, ICS.

Setting the Linux box up with a static IP is easy. The hard part is making sure Windows can still see it.

Right now I'm fixing a neat little dist-upgrade fuckup I made (I upgrade'd instead of dist-upgrade, big boo-boo) so I can't try it, though.

Amazingly, it worked just fine.

If your Linux machine has two NICs, you could just make it be the routing device instead of buying a dedicated one (assuming you don't have some more complex goal in mind than to stop using XP for routing).  You'll need the iptables package, a DHCP daemon, and Netfilter support in your kernel.  There are plenty of "for newbies" guides to getting this going, and it is quite quick to configure once you understand the principles.  Hit up your favorite Linux forums if you need help configuring it.  It is a pretty common configuration, so there are lots of people to guide you.