Can't help you with the Ethernet connection, but there are several options for adding connectivity. Of course you may be able to do the 3.5mm input jack mod, and just use it like that. However here is what I'm looking at
http://www.autotoys.com/x/product.php?productid=26516. It has two 3.5mm inputs (that can be used as is or used for the acc HD radio or SAT radio inputs), Blue Tooth, USB. It also includes a Mic for the Blue Tooth phone connection. The thing is pretty freaking cool if you ask me!
Now here's the kicker I'm almost 100% sure you have to have the XM compatible radio for this to work. From what I've gathered you may have the XM radio even if your rig didn't come with XM (There will be an additional multi pin plug on the back of the head unit, and I read somewhere on here what exactly to look for, but I can't find it now). I may be completely wrong on this, but I'm thinking that if you have any of the OEM stand alone extras (CD changer, DVD, console tape deck, ect...). You have the required headunit. One other issue I have heard that may arise is having to take the truck to a GM service center and have them "activate" the input interface on the OEM unit, but I'm hoping that only applies if you have the XM headunit, and no other stand alone extras. My research isn't done on this whole project. Once I figure out some concrete facts I plan on doing some sort of write up....
http://www.autotoys.com/x/product.php?productid=26516