No, this is not possible without jailbreaking.
If you do jailbreak though, you can purchase and download an app called MyWi from the ModMyi repository. If you're not interested in paying for the application, you could always download the cracked version from the SiNfuL iPhone repository, although you "should" support the people who developed it by purchasing it.
Doesn't have much use on an iPod though, seeing you must be connected to a network in the first place, unlike the iPhone where you have your 3G network.