I’ve seen both, even in the same Wikipedia article entry. Is there a right and wrong version, or is either version OK as long as I’m consistent?
I’m using the word in the context of patent law, as in ‘this invention must do A, B, and C to be considered novel and non(-)obvious’.
In US patent law (PDF) they use non-obvious.
103 Conditions for patentability; non-obvious subject matter.
In UK patent law (PDF) they use not obvious.
An invention shall be taken to involve an inventive step if it is not obvious to a person skilled in the art…
New Zealand uses both non-obvious and not obvious.
The objective of the international preliminary examination is to formulate a preliminary and non-binding opinion on the questions whether the claimed invention appears to be novel, to involve an inventive step (to be non-obvious), and to be industrially applicable.
The description shall first state the title of the invention as appearing in the request and shall: …
(vi) indicate explicitly, when it is not obvious from the description or nature of the invention …
Canada (PDF) uses not be obvious:
28.3 Invention must not be obvious
Eire uses not obvious:
An invention is considered as involving an inventive step if it is not obvious to a person skilled in that area of technology…
From this sample I surmise that non-obvious is the form you want although you might consider not obvious.