I suggest adding a magic number for the NBT specification.
It would make it easier to see if a file is an NBT file, and if its compressed or not.
A Compressed version is going to have its compression algorithm's Magic number(1F 8B 08 for gzip)
I say for minecraft use:
00 00 4E 42 54 00 00 xx
So it looks like
.. .. N B T .. .. xx
xx could just be 00, or it could be a version, in case Notch needs to change the specification down the road.
When reading in the file, all you have to do is check the first 8 bytes and see if it equals.
(if xx is 00) 86046784094208
(if xx is 01) 86046784094209
(if xx is 02) 86046784094210
(if xx is 10) 86046784094224
Also one could just open the file in notepad, and they would see NBT near the top.