So many things wrong with these posts. I feel obligated to clear them up.
Let us start with some random mistakes...
"If you're talking about using this in someone else's plane, keep in mind that the regs prohibiting cell phone usage are actually regs prohibiting radio receivers and transmitters being used in the passenger compartment- it's just that cell phones are the most common example of such a thing."
This is wrong. This is completely wrong.
The "regs" regarding using a cell phone in a plane are FCC regulations, and they ONLY ban cell phone usage. They ban ALL cell phone usage in a plane(for communication issues). There are other FAA regulations(which I will not discuss), but the FCC regulations against cell phones in planes has nothing to do with safety of the plane. It is a bit complicated, but it involves a potential problem with cell systems.
NO ONE can use a cell phone in a plane(including the pilot). Many small aircraft pilots have been known to use a cell, but they are technically in violation of FCC regs
However, the pilot uses all kinds of approved communications equipment(but not a cell).
Next....
"I'd like to be able to use my phone in an airplane or boat where there is no cell signal and still get live radar. I'm not sure if this is possible but it does seem plausible"
It isn't plausible..
GPS transmits at 50bit/s
Let me put it to you this way. Let us say you wanted to download a 100kb picture(size of a picture that would fill your screen(assuming compression)) It would take half an hour to receive the picture. Also, in that half hour, the GPS satellites wouldn't be sending any time data(which is how you determine your position.
GPS is simply a clock. It does not broadcast any additional information.
"I wouldn't mind even using some kind of USB adapter (or Bluetooth) to connect to a seperate receiver if needed to get this info from either GPS or even an XM/Sirius signal. Can anyone point me in the right direction for a way to view live weather radar on my phone in an area where there is no cell signal?"
You could use a satellite phone
but you probably just want to purchase a device specifically designed for this type of stuff.
try
http://www.sirius.com/marineweather
or visit your local marine outfitter(he can probably tell you more than I can)
At the end of the day, there is only so much you can do with a radio or antenna. They aren't magical devices, they are simply electrical machines. Asking if GPS can suddenly pick up weather radar is a bit like asking if you can make your bicycle fly.
Hope that was informative for everyone