Of course, you cannot share an Internet connection using the same port you're accessing the Web on. For example, you cannot share through an AirPort if you connect to the Internet using AirPort.
I'm not sure that this is right. For example I know that I can share an internet connection on my built-in ethernet port with others via that same port.
It works like this:
Laptop -> hub -> uni network
computer wanting to share
Basically the laptop (me) is the only one that the uni network will route packets for (does MAC address authentication at the DHCP server), so I route those packets on behalf of the other computer.
Unix boxes can run two interfaces through the one ethernet card---keeping the 'external interface IP' while dishing out 10.0.0.0 addresses to other clients.
The only danger here is that you are running an unauthorized DHCP server on the university network which could cause serious problems as other people that would normally pick up their IP from the uni DHCP servers might get one from you (and therefore appear to the internet as your public IP address. Actually for basic browsing this won't be a huge prob but could cause unexpected issues . Turning turning off the sharing will cause the accidentlly connected clients to hiccup ---they will need to re-new their DHCP lease from the right server. Also your sysadmins might well beat you up.
Note that, wierdly, this also works for "Sharing Airport over Airport"---but only if the network that you are connecting to is an open connect one that the other client can associate with---in this case the Airport network is working as the Hub. At least I have this option when I attach to an Ad-hoc network. What you can't do with Airport-to-Airport is run two channels, with one being the external interface and one the internal interface (the cards can't do it).
I haven't tested all this but it is worth keeping in mind. Again the DHCP issues could be serious.