Do you own and control the router to which your iPod touch is connecting to the network? If so, then in all likelihood you already have only one "real" IP address, belonging to the router itself. All the devices behind the router, including the iPod touch, are using "internal" IP addresses that are not visible to the outside world.
Whenever one of these devices requests something from the Internet, the router acts as an intermediary. It interprets the requests and repeats them to the outside world, but it uses its own "real" IP address as the return address instead of the internal device's address. Then, when a response arrives, the router remembers which internal device had originated each connection, and forwards the responses accordingly.
If this accurately describes your set-up, then it is absolutely possible, with most routers, to manually set aside a certain range of "internal" IP addresses for static allocation. However, the static addresses would still be "internal" only - not visible beyond the confines of the router you've manually configured. As well, you must never allow any two internal devices to attempt to use the same IP address. It is never appropriate to use any of the IP addresses that have been reserved for dynamic allocation (via DHCP) as static addresses.
Out of curiosity: Why do you need this? I'm finding it difficult to think of circumstances in which it would be meaningful for an iPod touch to need a static IP address. Normally static IP addresses only come in handy for software that's acting as a server (so that other computers know how to reach it), and normally server software wouldn't run on an iPod touch.
