It's hard to say what the issue is without knowing what wifi card your netbook uses (or at least what netbook it is, which would narrow that down to one or two different choices)
As everyone else said it's probably a driver issue. If you have a Broadcom based wifi card it is probably because you haven't chosen which driver you want, the BT43 open-source driver or the Broadcom STA driver.
You would have to plug in the Ethernet card to connect you up to the net and then run the System->Administration->Hardware Drivers control panel and choose which one you want. Ubuntu will then download it.
Last edited by rodmacpherson
on Sat Jun 12, 2010 6:16 pm, edited 1 time in total.