The Dell Vostro 3300 laptop ships with the Dell Wireless 1397 WLAN Mini-Card. This is not detected automatically by Ubuntu and wireless is not enabled.
The Dell Wireless card is nothing but a Broadcom card. To enable wireless, choose System → Administration → Hardware Drivers. The system will now search for required proprietary drivers. In the list it presents, choose the Broadcom STA Wireless Driver. After it is installed, reboot and your wireless card should now work.
Tried with: Ubuntu 10.04 (Lucid Lynx)