Has anyone tried these systems? With these I guess it bypasses the safety devices where you can watch DVD and have full navigation function while driving? Are there any other brands?

http://www.gnetcanada.com/navvideo-models-gmunlock.asp

works great! got mine for $250 Florida company. costal---something

I'm guessing you mean Coastal Technologies? I'd be careful buying anything from them...sounds like you got lucky.
I wanted one of their "lockpicks" for my Lexus until I read the comments on a Club Lexus message board. Lots of horror stories there about defective product, horrible customer service, and even other electronics in the vehicle not working correctly until the lockpick was removed.

Do you have factory or aftermarket unit?

I have an 07 tahoe and just put in the Kenwood DNX7100 and had all the goodies put in. Bluetooth, steering wheel controls, XM, EVERYTHING!!!!! The local stereo place i use just "showed" me how to bypass the system. Basically left an wire hanging that needed to be grounded.

I have also heard you can put the parking brake down one click and it will bypass the system?

yep ground it and push the P brake in till it clicks once. We have done a couple hundred of these when i worked for a car audio shop in La.