I've never done so before, but I believe this is indeed possible.
Firstly, you need to open "System Preferences" and then click on "Sharing". Then, navigate to the "Internet" section and select check the box next to "Airport". Next, click on "Airport Options" and then type a name for the wireless network that your computer will create. You may configure the security settings as you wish and click "OK". Finally, click on "Start", below "Internet Sharing" to launch the network you will connect to via your iPod touch. Now all that is left for you to do is to connect to this network via your iPod touch, and theoretically you should be able to connect to the internet via the network you have just created.
I apologise if this does not work, because as noted earlier, I have never used this feature!
