said by JohnInSJ:
It is a violation of the TOS to have an open wifi. It is also a poor defense in a legal case. However, and this is the MOST IMPORTANT POINT, Comcast isn't going to sue the OP, they will after 5 (or 6?) of these letters simply cut off his interent for a year. Without trial. Without appeal. Because they can.
So personally I would not throw away the letter, I would secure my wifi.
Speaking of securing something the ISP fails to tell you is it will slow down your surfing from wireless connections. So if the wife is in the living room on her laptop and the router is in the bedroom and each and every packet she is sending to and from the router has to be encrypted and decrypted....running unsecured is a viable option to speed up that connection. Securing a WI-FI signal can rob you anywhere from 5% to 30% of throughput.