This is a popular fan theory (Jon's parentage is very ambiguous, and it can be argued that Tyrion's parentage is not completely clear, leading to speculation that they both have Targaryen blood).
At the same time, it would be sort of lame if GRRM decided "well, everyone figured out R+L=J and the three headed dragon and all so I guess I better change some things around".
Then I just get a little bit bummed out (and guilty for feeling entitled) that the next book probably won't be out for a while still.