The thing you've all been waiting for: The Na'vi Cryptogram Generator
Basically, it generates a cryptogram (simple substitution cypher, as found on the funnies pages of your newspaper) based on the letters we are using to write Na'vi.
What's it good for? I don't know
No, seriously, when I was learning English, solving cryptograms helped me a lot to develop a "feel" for the structure of the written language. That being said, whether it's actually that useful in Na'vi - I highly doubt, but it might just be good fun.
It should be pretty self-explanatory to use, but please, let me know if there's something weird happening or you'd like something changed. Also, the usual: Please don't try to break it.