Can the theft deterrent system on GM cars be by-passed?

Supposedly there are bypass modules....some are so that the remote start systems can function with the security system in the car....others are just strictly to bypass the security system intirely. I have seen some websites that sell these modules such as Ebay but there are some more specific car sites that sell them too. I have read a few things on this recently and it looks as though GM has used the Passkey I, II and III systems. The year, make and model of the car depends on what system was used and what type of bypass you will need.