This is a common problem in Ubuntu. The led which shows wlan activity doesn’t blink (flicker) at all. I googled and found some results out of which this tutorial helped a lot:

http://madwifi-project.org/wiki/UserDocs/EnableLEDs

This maybe due to the fact that I am using madwifi drivers for my wlan. The other solution that I found was, though it didn’t work on mine but it has worked for many others:
http://knowledge76.com/index.php/Wireless_LED_on_Ubuntu

Cheers!