If you have installed Ubuntu Feisty beta on your SI1520:
DO NOT INSTALL TODAY'S UPDATES (April 11).
Something is wrong with the update, and the network manager stops working. As a consequence, network adapters are not recognized anymore, the wireless IPW3945 will not work, and the "cabled" network will work only after setting it up again (but you will not have any confirmation from the network manager icon, since this won't work).
You have been warned! ;)
Until the problem is solved, it is possible to downgrade the network manager to the previous (working) version:
sudo dpkg -i /var/cache/apt/archives/
(please notice: it's a single line of code)
This will work if you still have the package in your cache.