After setting up your google voice account and phone number, dial the following code to forward your number.
Wireless Carrier or Network —> Activation Code
AT&T Cingular, T-Mobile, GSM network: *004*1[GVnumber]*11#
Verizon (CDMA network): *71[GVnumber] AND *90[GVnumber] AND *92[GVnumber]
Verizon (TDMA Network): *74[GVnumber] and *73[GVnumber]
Bluegrass Cellular: *90#[GVnumber] and *92#[GVnumber]
Cellcom: *68[GVnumber]
Cincinnati Bell: *004*[GVnumber]#
US Cellular: *74[GVnumber]
Reference: Google Product Page