I recently install Debian etch (2.6.18-6) on my lenovo T61. I installed ieee80211 subsystem and ipw3945 module for the intel 3945abg wireless card. All modules and daemon are sucessful installed. No problem in dmesg, lspci output or modprobe these modules. The output of iwconfig showed the correct wireless interface.

I set up the correct id and key corresponding to my access point (AP) either by iwconfig or network-admin. Everything went smoothly, however, no access to the internet!!

I didn't see any error in any logs and I can see all the avaible APs nearby in the network-admin tool dialog. No matter which AP(either key or keyless) I chose and whether dhcp or static, no internet!

Could you please let me know if there are anythings I could do? Thank you!

Did you install the firmware?
Go down to Hardware-specific Notes for more info;
http://www.debian.org/releases/etch/etchnhalf

I finally stopped struggling with debian etch and decided to go for debian lenny directly and to use iwlwifi instead of ipw3945 with kernel 2.6.26. Everything works find for me now except I can only make WPA but WEP working for wireless encryption. It's actually not a bad thing since WEP is insecure.