We're mostly there, as you can see, thanks to the voluntary efforts of Mikhail Korobov and others. There's some sticky issues with supporting Python 2 and 3 from the same codebase, issues with character encodings and unicode, and issues with our test framework. We're slowly resolving this, but depend on volunteers.
Now that NLTK has moved to GitHub, it is easier than ever for people to contribute new functionality and bugfixes. NLTK is downloaded nearly 300 times a day, so any contributions that people care to make are likely to have a significant impact.
Here are the places to start: https://github.com/nltk/nltk (the home of NLTK development) https://groups.google.com/forum/?fromgroups#!forum/nltk-dev (developers) https://groups.google.com/forum/?fromgroups#!forum/nltk-users (end users)