I'm a pilot at Scare,....er, ..... Air Canada and I double checked our manual about this one. They are allowed on our airplanes, but there is a cautionary note warning that there have been rare occurances of reported interference caused by "satellite receivers". A GPS would only be allowed to be turned on in cruise flight. Many airplanes, including some Air Canada planes use GPS's to navigate. How well a portable one will work in the back depends on the model, as I believe many portable ones will not keep up to the speeds of a jet in cruise flight, (450 to over 650 mph). Also, as noted by others, you may need to hold the unit up to the window to be able to recieve satellites.
As many flight attendants are clueless about what a GPS is, most will just say you can't use it, (tell them a pilot buddy said you can - its in the "Flight Operations Manual"...).
I don't know what other carriers have to say about GPS's, but officially at Air Canada - you can bring it on board and try it once we've levelled out. If the pilots notice strange things with the instruments, they'll send a message back to turn it off - A GPS won't crash the plane

CD players, MP3's, tape players, are all also OK, you just can't wear a headset for takeoff and landing - in case the Flight Attendants need to yell instructions, (if there is a need to get out of the plane in a hurry). The only things that are banned are things that send out IR signals, such as: a portable laptop printer, a remote control car, a Robosapien..... Also, with the ability to send and receive e-mails with Blackberry's and some cell phones, the send and recieve function of these units must be turned off if they are on in flight, (many people turn them on for games, schedules, etc).
Hope this helps,
Ron